mips: ralink: mt7620: remove clock related code

A proper clock driver for ralink SoCs has been added. Hence there is no
need to have clock related code in 'arch/mips/ralink' folder anymore.
Since this is the last clock related code removal, remove also remaining
prototypes in 'common.h' header file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>