dt-bindings: phy: rcar-gen2: Add r8a77470 support

Add USB PHY support for r8a77470 SoC. Renesas RZ/G1C (R8A77470)
USB PHY is similar to the R-Car Gen2 family, but has the below
feature compared to other RZ/G1 and R-Car Gen2/3 SoCs

It has a shared pll reset for usbphy0/usbphy1 and this register
reside in usbphy0 block.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>