Add API to return memory map on ARM platforms

Add API to return memory map on ARM platforms

Functions to configure the MMU in S-EL1 and EL3 on ARM platforms
expect each platform to export its memory map in the `plat_arm_mmap`
data structure. This approach does not scale well in case the memory
map cannot be determined until runtime. To cater for this possibility,
this patch introduces the plat_arm_get_mmap() API. It returns a
reference to the `plat_arm_mmap` by default but can be overridden
by a platform if required.

Change-Id: Idae6ad8fdf40cdddcd8b992abc188455fa047c74