plat(common): move platform independant dram manager

DRAM management is not platform specific. Move it under
plat/common directory to allow access of these common routines
to other platforms.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: I4362aeb7be8ad031ec802ebe53573d0070acbfc7