MIPS: ralink: mt7620: Add cpu-feature-override header

Override certain CPU features to help GCC to optimize
the generated code. Saves about 150KB in the vmlinux
image with a generic configuration.

     text    data     bss     dec     hex filename
  3824158  134820  234192 4193170  3ffb92 vmlinux.no-override
  3664054  138804  234192 4037050  3d99ba vmlinux.override

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5759/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>