microblaze: u-boot.lds: replace __end symbol with _end

microblaze: u-boot.lds: replace __end symbol with _end

board_fdt_blob_setup() uses the _end symbol to find the dtb in the non-spl
case. In order to allow microblaze builds to compile successfully with
CONFIG_OF_SEPARATE, the _end symbol must be defined. Align microblaze with
the other architectures and use _end symbol rather than __end to mark the
end of the u-boot binary.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211130163358.2531677-2-ovidiu.panait@windriver.com
Signed-off-by: Michal Simek <michal.simek@xilinx.com>