ARM: shmobile: r8a7794: add USB PHY DT support

Define the R8A7794 generic part of the USB PHY device node. It is up to the
board file to enable the device.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>