mips_malta: provide ememsize env variable to kernels

mips_malta: provide ememsize env variable to kernels

Commit 94c2b6aff43c (mips_malta: support up to 2GiB RAM) provided
support for using over 256MB of RAM with the MIPS Malta board, including
capping the memsize variable that QEMUs pseudo-bootloader provides to
the kernel at 256MB in order to match YAMON. It didn't however provide
the ememsize variable which kernels supporting memory outside of the
unmapped address spaces (ie. EVA or highmem) may use to determine the
true size of the RAM present in the system.

Set ememsize to the size of RAM so that such kernels may use all
available memory without the user having to manually specifying its size
& location.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>