clk: renesas: r9a06g032: Add missing PCI USB clock

The clock driver is missing support for the clk_pci_usb clock that is
present on the SoC. This is added to allow the clock to be supported.

Signed-off-by: Gareth Williams <gareth.williams.jx@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>