microblaze: u-boot-spl.lds: Pass _image_binary_end

Pass _image_binary_end to make a standard way to indicate the end
of the text section in SPL.

The motivation for this is to have a uniform way to handle
the SPL boundary checks.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>