arm64: define _image_binary_end to fix SPL_OF_CONTROL

To make SPL_OF_CONTROL work on ARM64 SoCs, _image_binary_end must be
defined in the linker script.

 LD      spl/u-boot-spl
lib/built-in.o: In function `fdtdec_setup':
lib/fdtdec.c:1186: undefined reference to `_image_binary_end'
lib/fdtdec.c:1186: undefined reference to `_image_binary_end'
make[1]: *** [spl/u-boot-spl] Error 1
make: *** [spl/u-boot-spl] Error 2

Note:
CONFIG_SPL_SEPARATE_BSS must be defined as well on ARM64 SoCs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>