ramips: enable CPS for mt7621

Enables CPS multiprocessing instead ob obsoleted CMP for mt7621.

This patch fixes a few issues currently existing on 4.3 kernel with at least ubnt-erx:
* iperf shows only 50Mbits on direct gigabit connection to desktop,
* ping times jump to 5-6ms to dorectly connected desktop
* /proc/interrupts shows spurious interrups (ERR)

Signed-off-by: Nikolay Martynov <mar.kolya@gmail.com>

SVN-Revision: 47842