ARM: shmobile: r8a7790: Add PCI USB host clock support

This adds internal PCI USB host clock support.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>