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>