ARM: shmobile: r8a7791: add HS-USB device node

Define the R8A7791 generic part of the HS-USB device node. It is up to the board
file to enable the device.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
[Sergei: fixed summary, added changelog]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>