meminfo cmd: Enable printing >=4GiB SDRAM sizes

gd->ram_size is stored as phys_size_t type which can be bigger than an
unsigned long on some architectures.  When using unsigned long type,
SDRAM of 4GiB or larger will not print the correct size, but using
phys_size_t will.

Signed-off-by: Andrew Bradford <andrew.bradford@kodakalaris.com>