clk: meson-pll: add reduced specific clk_ops for G12A PCIe PLL

The Meson G12A PCIE PLL is fined tuned to deliver a very precise
100MHz reference clock for the PCIe Analog PHY, and thus requires
a strict register sequence to enable the PLL.
To simplify, use the _init() op to enable the PLL and keep
the other ops except set_rate since the rate is fixed.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20190307141455.23879-2-narmstrong@baylibre.com