mips: Always permit to build u-boot images

The platforms where the kernel should be loaded above 0x8000.0000 do not
support loading u-boot images, that doesn't mean that we shouldn't be
able to generate them.

Additionally, since commit 79876cc1 ("MIPS: new Kconfig option
ZBOOT_LOAD_ADDRESS"), the $(zload-y) variable was no longer hardcoded,
which made it impossible to use the uzImage.bin target.

Fixes: 79876cc1 ("MIPS: new Kconfig option ZBOOT_LOAD_ADDRESS")
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>