MIPS: malta: malta-memory: Add support for the 'ememsize' variable

The 'ememsize' variable is used to denote the real RAM which is
present on the Malta board. This is different compared to 'memsize'
which is capped to 256MB. The 'ememsize' is used to get the actual
physical memory when setting up the Malta memory layout. This only
makes sense in case the core operates in the EVA mode, and it's
ignored otherwise.

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