ARM: shmobile: r8a7794: add HS-USB DT support

Define the R8A7794 generic part of the HS-USB 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>