ARM: dts: imx: Enable PCIe and NVMe on DH i.MX8M Plus DHCOM PDK3

ARM: dts: imx: Enable PCIe and NVMe on DH i.MX8M Plus DHCOM PDK3

Enable PCIe/NVMe support on DH i.MX8M Plus DHCOM PDK3. Except for
the configuration options which are enabled, add slight adjustment
to board u-boot.dtsi, which is necessary as there is currently no
driver for the I2C PCIe clock generator. Since the generator is
strapped to be always on, it is possible to supplant the generator
functionality by fixed-clock.

Signed-off-by: Marek Vasut <marex@denx.de>