ARM: shmobile: sh73a0: add support for adjusting CPU frequency

On SH73A0 the output of PLL0 is supplied to two dividers, feeding clock to
the CPU core and SGX. Lower CPU frequencies allow the use of lower supply
voltages and thus reduce power consumption.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>