ramips: fix cpu clock of mt7621 and add dt clk devices

ramips: fix cpu clock of mt7621 and add dt clk devices

For a long time the mt7621 uses a fixed cpu clock which causes a problem
if the cpu frequency is not 880MHz.

This patch fixes the cpu clock calculation and adds the cpu/bus clkdev
which will be used in dts.

Signed-off-by: Weijie Gao <hackpascal@gmail.com>