common: Add get_effective_memsize() to memsize.c

This function has been around for powerpc. It is used for systems with
memory more than CONFIG_MAX_MEM_MAPPED. In case of non-contiguous memory,
this feature can limit U-boot to one block without going over the limit.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>