hw/mips_malta: Fix KVM PC initialisation

Commit 71c199c8 ("mips_malta: provide ememsize env variable to
kernels") changed the meaning of loaderparams.ram_size to be the whole
of RAM rather than just the low part below where the boot code is placed
for KVM, but it didn't update the PC initialisation for KVM to use
ram_low_size. Fix that now.

Fixes: 71c199c8 ("mips_malta: provide ememsize env variable to kernels")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>