arm64: dts: r8a7795: salvator-x: enable usb2_phy of channel 0

This patch also adds a regulator node for USB2.0 to handle VBUS on/off
by the phy-rcar-gen3-usb2 driver.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>