ARM: shmobile: r8a7778: add usb phy power control function

USB phy initialisation function is needed from not only
USB Host but also USB Function too.
This patch adds usb phy common control function.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>