ARM: OMAP2+: Move stubbed secure_sram_reserve function to a common.c and call it __weak

omap_secure_ram_reserve_memblock is stubbed for OMAP1,2 only builds using a
 ifdef check. But this results in adding CONFIG_ARCH_OMAPxx checks for
future socs that use the real function. So move this to common.c file and
call it __weak.

Signed-off-by: R Sricharan <r.sricharan@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>