ArmPlatformPkg: PL061: support multiple controller

ArmPlatformPkg: PL061: support multiple controller

Support multiple PL061 controllers. If platform gpio driver couldn't be
found, PL061 gpio driver will continue to load PcdPL061GpioBase as the
register base.

It could be compatible with the use case of current PL061 gpio driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>