MIPS: ralink: add MT7621 support

MT7621 is based on a 1004k core. This patch adds support for the SoC. The
timer and IRQ is just boiler plate as GIC has recently been moved to
generic places in the kernel and just works.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11990/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>