MIPS: bcm47xx: enable ZBOOT support

MIPS: bcm47xx: enable ZBOOT support

Enable ZBOOT support. The WRT54GL router's bootloader limits kernel
size to 3 MB with the normal load address, which is a bit challenging
vmlinux size with modern Linux. A compressed kernel allows booting
much bigger kernels.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18492/
Signed-off-by: James Hogan <jhogan@kernel.org>