MIPS: Alchemy: Turn on -Werror for devboards and xss1500

Warnings being suppressed, we can now turn on -Werror for boards which did
not have it already (devboards and xss1500).

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>