ARM: shmobile: sh73a0: add CPUFreq support

This patch enables the use of the generic cpufreq-cpu0 driver on sh73a0.
Providing a regulator, a list of OPPs in DT, combined with a virtual
cpufreq-cpu0 platform device and a clock, attached to it is everything,
the cpufreq-cpu0 driver needs. The first sh73a0 platform, implementing
such CPUFreq support is kzm9g-reference.

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