clk: renesas: rcar-usb2-clock-sel: Add R-Car USB 2.0 clock selector PHY

R-Car USB 2.0 controller can change the clock source from an oscillator
to an external clock via a register. So, this patch adds support
the clock source selector as a clock driver.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>