mips: enable support for appending dtb to spl binary

mips: enable support for appending dtb to spl binary

If CONFIG_SPL_OF_CONTROL is enabled for SPL and CONFIG_OF_SEPARATE is also
enabled, the dtb will be appended to the u-boot-spl.bin.

When calling dm_init_and_scan() in SPL, fdtdec_setup() will try to locate
dtb at the end of u-boot-spl.bin, by referencing to _image_binary_end.

However _image_binary_end is currently missing in u-boot-spl.lds.
This patch adds _image_binary_end to u-boot-spl.lds to make sure linking
u-boot-spl will not fail.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>