plat(common): provide flexibility to dram bank lookup

On some platforms, the complete avialable main memory can be composed
of more than 2 or multiple non-contiguous DRAM banks. Modify the common
handler to discover and map any number of DRAM banks to array of
GRANULES as one single contiguous chunk of memory.

Add a new option RMM_MAX_DRAM_NUM_BANK to choose number of supported
dram banks and update all the existing configs with appropriate default
value.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>