ARM: OMAP: PM: Add support to allocate the memory for secure RAM

Allocate the memory to save secure ram context which needs
to be done when MPU is hitting OFF mode.

The ROM code expects a physical address to this memory
and hence use memblock APIs to reserve this memory as part
of .reserve() callback. Maximum size as per secure RAM requirements
is allocated.

To keep omap1 build working, omap-secure.h file is created
under plat-omap directory.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Vishwanath BS <vishwanath.bs@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>