Presently the SH7720/21 serial code uses asm/gpio.h to get at the CPU
GPIO port definitions, but in the case of GPIOLIB=y this also includes
references to generic GPIOLIB routines that we don't have any function
declarations for, tripping up on -Werror=implicit-function-declaration
with newer gcc versions:

  CC      arch/sh/kernel/cpu/sh3/serial-sh7720.o
In file included from include/linux/sh_pfc.h:14:0,
                 from arch/sh/include/asm/gpio.h:23,
                 from arch/sh/kernel/cpu/sh3/serial-sh7720.c:5:
include/asm-generic/gpio.h: In function 'gpio_get_value_cansleep':
include/asm-generic/gpio.h:220:2: error: implicit declaration of function '__gpio_get_value' [-Werror=implicit-function-declaration]
include/asm-generic/gpio.h: In function 'gpio_set_value_cansleep':
include/asm-generic/gpio.h:226:2: error: implicit declaration of function '__gpio_set_value' [-Werror=implicit-function-declaration]
In file included from arch/sh/include/asm/gpio.h:23:0,
                 from arch/sh/kernel/cpu/sh3/serial-sh7720.c:5:
include/linux/sh_pfc.h: At top level:
include/linux/sh_pfc.h:121:19: error: field 'chip' has incomplete type

Switch to using the cpu/ version for the port definitions explicitly.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>