omap4: sram: Fix start address

On OMAP4 there is no need to have SRAM_BOOTLOADER_SZ provision

Hence put this macro under CONFIG_ARCH_OMAP2PLUS check

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>