Originally OMAP MPUIO GPIO irqchip was implemented using Generic irq
chip, but after set of reworks Generic irq chip code was replaced by
common OMAP GPIO implementation and finally removed by
commit d2d05c65 ("gpio: omap: Fix regression for MPUIO interrupts").
Unfortunately, above commit left .irq_mask/unmask callbacks assigned
as below for MPUIO GPIO case:
	irqc->irq_mask = irq_gc_mask_set_bit;
	irqc->irq_unmask = irq_gc_mask_clr_bit;

This now causes boot failure on OMAP1 platforms, after
commit 450fa54c ("gpio: omap: convert to use generic irq handler")
which forces these callbacks to be called during GPIO IRQs mapping
from gpiochip_irq_map:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c0004000
[00000000] *pgd=00000000
Internal error: Oops: 75 [#1] ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.4.0-rc1-e3-los_afe0c+-00002-g25379c0-dirty #1
Hardware name: Amstrad E3 (Delta)
task: c1836000 ti: c1838000 task.ti: c1838000
PC is at irq_gc_mask_set_bit+0x1c/0x60
LR is at __irq_do_set_handler+0x118/0x15c
pc : [<c004848c>]    lr : [<c0047d4c>]    psr: 600000d3
sp : c1839c90  ip : c1862c64  fp : c1839c9c
r10: 00000000  r9 : c0411950  r8 : c0411bbc
r7 : 00000000  r6 : c185c310  r5 : c00444e8  r4 : c185c300
r3 : c1854b50  r2 : 00000000  r1 : 00000000  r0 : c185c310
Flags: nZCv  IRQs off  FIQs off  Mode SVC_32  ISA ARM  Segment kernel
Control: 0000317f  Table: 10004000  DAC: 00000057
Process swapper (pid: 1, stack limit = 0xc1838190)
Stack: (0xc1839c90 to 0xc183a000)

[...]

Backtrace:
[<c0048470>] (irq_gc_mask_set_bit) from [<c0047d4c>] (__irq_do_set_handler+0x118/0x15c)
[<c0047c34>] (__irq_do_set_handler) from [<c0047dd4>] (__irq_set_handler+0x44/0x5c)
 r6:00000000 r5:c00444e8 r4:c185c300
[<c0047d90>] (__irq_set_handler) from [<c0047e1c>] (irq_set_chip_and_handler_name+0x30/0x34)
 r7:00000050 r6:00000000 r5:c00444e8 r4:00000050
[<c0047dec>] (irq_set_chip_and_handler_name) from [<c01b345c>] (gpiochip_irq_map+0x3c/0x8c)
 r7:00000050 r6:00000000 r5:00000050 r4:c1862c64
[<c01b3420>] (gpiochip_irq_map) from [<c0049670>] (irq_domain_associate+0x7c/0x1c4)
 r5:c185c310 r4:c185cb00
[<c00495f4>] (irq_domain_associate) from [<c0049894>] (irq_domain_add_simple+0x98/0xc0)
 r8:c0411bbc r7:c185cb00 r6:00000050 r5:00000010 r4:00000001
[<c00497fc>] (irq_domain_add_simple) from [<c01b3328>] (_gpiochip_irqchip_add+0x64/0x10c)
 r7:c1862c64 r6:c0419280 r5:c1862c64 r4:c1854b50
[<c01b32c4>] (_gpiochip_irqchip_add) from [<c01b79f4>] (omap_gpio_probe+0x2fc/0x63c)
 r5:c1854b50 r4:c1862c10
[<c01b76f8>] (omap_gpio_probe) from [<c01fcf58>] (platform_drv_probe+0x2c/0x64)
 r10:00000000 r9:c03e45e8 r8:00000000 r7:c0419294 r6:c0411984 r5:c0419294
 r4:c0411950
[<c01fcf2c>] (platform_drv_probe) from [<c01fb668>] (really_probe+0x160/0x29c)

Hence, fix it by remove obsolete callbacks assignment. After this
change 	omap_gpio_mask_irq()/omap_gpio_unmask_irq() will be used
for MPUIO IRQs masking, but this now happens anyway from
omap_gpio_irq_startup/shutdown().

Cc: Tony Lindgren <tony@atomide.com>
Fixes: commit d2d05c65 ("gpio: omap: Fix regression for MPUIO interrupts")
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>