counter: Add Renesas RZ/G2L MTU3a counter driver

Add RZ/G2L MTU3a counter driver. This IP supports the following
phase counting modes on MTU1 and MTU2 channels

1) 16-bit phase counting modes on MTU1 and MTU2 channels.
2) 32-bit phase counting mode by cascading MTU1 and MTU2 channels.

This patch adds 3 counter value channels.
	count0: 16-bit phase counter value channel on MTU1
	count1: 16-bit phase counter value channel on MTU2
	count2: 32-bit phase counter value channel by cascading
                MTU1 and MTU2 channels.

The external input phase clock pin for the counter value channels
are as follows:
	count0: "MTCLKA-MTCLKB"
	count1: "MTCLKA-MTCLKB" or "MTCLKC-MTCLKD"
	count2: "MTCLKA-MTCLKB" or "MTCLKC-MTCLKD"

Use the sysfs variable "external_input_phase_clock_select" to select the
external input phase clock pin and "cascade_counts_enable" to enable/
disable cascading of channels.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: William Breathitt Gray <william.gray@linaro.org>
Acked-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230330111632.169434-5-biju.das.jz@bp.renesas.com

drivers/counter/Kconfig

@@ -73,6 +73,17 @@ config MICROCHIP_TCB_CAPTURE
 	  To compile this driver as a module, choose M here: the
 	  module will be called microchip-tcb-capture.
 
+config RZ_MTU3_CNT
+	tristate "Renesas RZ/G2L MTU3a counter driver"
+	depends on RZ_MTU3 || COMPILE_TEST
+	help
+	  Enable support for MTU3a counter driver found on Renesas RZ/G2L alike
+	  SoCs. This IP supports both 16-bit and 32-bit phase counting mode
+	  support.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called rz-mtu3-cnt.
+
 config STM32_LPTIMER_CNT
 	tristate "STM32 LP Timer encoder counter driver"
 	depends on MFD_STM32_LPTIMER || COMPILE_TEST

drivers/counter/Makefile

@@ -8,6 +8,7 @@ counter-y := counter-core.o counter-sysfs.o counter-chrdev.o
 
 obj-$(CONFIG_104_QUAD_8)	+= 104-quad-8.o
 obj-$(CONFIG_INTERRUPT_CNT)		+= interrupt-cnt.o
+obj-$(CONFIG_RZ_MTU3_CNT)	+= rz-mtu3-cnt.o
 obj-$(CONFIG_STM32_TIMER_CNT)	+= stm32-timer-cnt.o
 obj-$(CONFIG_STM32_LPTIMER_CNT)	+= stm32-lptimer-cnt.o
 obj-$(CONFIG_TI_EQEP)		+= ti-eqep.o