MIPS: BCM47xx: Select GPIOLIB for BCMA on bcm47xx platform

The Kconfig items BCM47XX_BCMA and BCM47XX_SSB selected respectively
BCMA_DRIVER_GPIO and SSB_DRIVER_GPIO. These options depend on GPIOLIB
without explicitly selecting it so it results in a warning when GPIOLIB
is not set:

scripts/kconfig/conf --oldconfig Kconfig
warning: (BCM47XX_BCMA) selects BCMA_DRIVER_GPIO ... unmet direct
	dependencies (BCMA_POSSIBLE && BCMA && GPIOLIB)
warning: (BCM47XX_SSB) selects SSB_DRIVER_GPIO ... unmet direct
	dependencies (SSB_POSSIBLE && SSB && GPIOLIB)

which subsequently results in compile errors.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4759/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>