plat(common): provide flexibility in dram bank lookup

Next generation infrastructure reference design platform such as RD-Fremont
can have more than 2 non-contiguous DRAM banks. On such platforms the DRAM
banks are spread across the system memory map. So modify the common handler
to discover and map any number of DRAM banks to an array of granules as one
single contiguous chunk of memory.
Add the new config: RMM_MAX_NUM_DRAM_BANK to define the maximum number of
DRAM banks supported by a given platform.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Change-Id: I5f9ab35a820642132e77fb5acde216bb321991e8