clk: meson-g12a: add PCIE PLL clocks

Add the PCIe reference clock feeding the USB3 + PCIE combo PHY.

This PLL needs a very precise register sequence to permit to be locked,
thus using the specific clk-pll pcie ops.

The PLL is then followed by :
- a fixed /2 divider
- a 5-bit 1-based divider
- a final /2 divider

This reference clock is fixed to 100MHz, thus only a single PLL setup
is added.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20190307141455.23879-4-narmstrong@baylibre.com