arm64: dts: imx8mp: Describe PCIe clock generator on DH electronics i.MX8M Plus DHCOM on PDK3

The PDK3 carrier board contains a PCIe clock generator which is used to
supply the PCIe clock lanes. This generator is always on, unless external
CLKREQ signal toggles an output off, but this is handled in hardware. The
generator does however have I2C interface, describe it in DT.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>