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>

README

@@ -3869,6 +3869,14 @@ Configuration Settings:
 		Scratch address used by the alternate memory test
 		You only need to set this if address zero isn't writeable
 
+- CONFIG_SYS_MEM_RESERVE_SECURE
+		If defined, the size of CONFIG_SYS_MEM_RESERVE_SECURE memory
+		is substracted from total RAM and won't be reported to OS.
+		This memory can be used as secure memory. A variable
+		gd->secure_ram is used to track the location. In systems
+		the RAM base is not zero, or RAM is divided into banks,
+		this variable needs to be recalcuated to get the address.
+
 - CONFIG_SYS_MEM_TOP_HIDE (PPC only):
 		If CONFIG_SYS_MEM_TOP_HIDE is defined in the board config header,
 		this specified memory area will get subtracted from the top