Document RZ/G1C (R8A77470) SoC bindings.

For RZ/G1C, this driver is used to enable interrupt generation and
initializing timing registers which is part of phy_init code.
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>