clk: meson-axg: pcie: drop the mpll3 clock parent

We found the PCIe driver doesn't really work with
the mpll3 clock which is actually reserved for debug,
So drop it from the mux list.

Fixes: 33b89db68236 ("clk: meson-axg: add clocks required by pcie driver")
Tested-by: Jianxin Qin <jianxin.qin@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>