ARM: shmobile: sh73a0: add USB clock support

It is required from renesas_usbhs driver.
This patch is based on v2.0 manual.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>