arm64: dts: imx8mp: move PCIe controller clock config to SoC dtsi

The only difference in PCIe clock configuration between boards is how
the PCIe reference clock is generated. The refclock configuration is
fully contained in the PCIe PHY node, so the PCIe controller clocks
can be set up in the SoC dtsi, as there is no reason for any board to
use a different configuration.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>