plat(common): move to a platform-independent DRAM manager

DRAM management is not platform-specific. Move it under plat/common
instead so it can be utilised by all platforms.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Change-Id: I668c07e7e042aba57f82cb069f79a111823ef0a0