ARM: shmobile: r8a7794: link PCI USB devices to USB PHY

Describe the PCI USB devices that are behind the PCI bridges, adding necessary
links to the USB PHY device.

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