ARM: shmobile: add cpufreq-cpu0 driver for common SH-Mobile

I add a new file(cpufreq.c) for the following reasons.
- Registration of platform_device must be unified in SH-Mobile.
- We can't create a node of cpufreq drivers into device tree.
  (Because cpufreq driver is virtual device.)

Signed-off-by: Gaku Inami <gaku.inami.xw@bp.renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>