Document: add PLAT_PL061_MAX_GPIOS define
Document: add PLAT_PL061_MAX_GPIOS define

ARM PL061 GPIO driver requires the "PLAT_PL061_MAX_GPIOS" definition.
By default, it's defined to 32 in PL061 GPIO driver. If user wants
more PL061 controllers in platform, user should define the build
flag in platform.mk instead.

Signed-off-by: Haojian Zhuang <979139d45d262f0480136b0da512e0afa0fe79d3@linaro.org>

davwan01/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,davwan01/arm-trusted-firmware,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware