MIPS: Allow platforms to specify the decompressor load address

Platforms which use raw zboot images may need to link the image at
a fixed address if there is no other way to communicate the load
address to the bootloader.  Allow the per-platform Kbuild files
to specify an optional zboot image load address (zload-y) and fall
back to calc_vmlinuz_load_addr if unset.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: James Hartley <james.hartley@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9566/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>