MIPS: malta: malta-memory: Use the PHYS_OFFSET to build the memory map

PHYS_OFFSET is used to denote the physical start address of the
first bank of RAM. When the Malta board is in EVA mode, the physical
start address of RAM is shifted to 0x80000000 so it's necessary to use
this macro in order to make the code EVA agnostic.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>