Reserve secure memory

Secure memory is at the end of memory, separated and reserved
from OS, tracked by gd->secure_ram. Secure memory can host
MMU tables, security monitor, etc. This is different from PRAM
used to reserve private memory. PRAM offers memory at the top
of u-boot memory, not necessarily the real end of memory for
systems with very large DDR. Using the end of memory simplifies
MMU setup and avoid memory fragmentation.

"bdinfo" command shows gd->secure_ram value if this memory is
marked as secured.

Signed-off-by: York Sun <yorksun@freescale.com>