clk: meson-axg: add pcie and mipi clock bindings

Add the pcie and mipi clock dt-bindings for the pcie driver.

Since the mipi clock isalso used by the pcie driver,
we add it together in this patch.

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>