MIPS: Add -Werror to arch/mips/Kbuild

Adding subdirs-ccflags-y := -Werror to arch/mips/Kbuild
let us in one go cover all files with -Werror.

In addition this allows us to remove the
individual -Werror definition in various Makefile.

Adding the definition to Kbuild as a recursive
option help us not to forget to do so.

With this change we now compile arch/mips/kernel/cpufreq with -Werror

One drawback:
When specifying a subdirectory covered by the Kbuild file like this:

    make arch/mips/kernel/

then kbuild fails to pick up the -Werror definition.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
To: linux-mips <linux-mips@linux-mips.org>
To: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1301/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>