ARM: OMAP2+: define _image_binary_end to fix SPL_OF_CONTROL

To make SPL_OF_CONTROL work on OMAP2+ SoCs, _image_binary_end must be
defined in the linker script along with CONFIG_SPL_SEPARATE_BSS.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>