dt-bindings: clk: g12a-clkc: add PCIE PLL clock ID

Add a clock ID for the reference clock feeding the USB3+PCIe Combo PHY.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20190307141455.23879-3-narmstrong@baylibre.com