arm64: dts: imx8mq: assign PCIe clocks

This fixes multiple issues with the current non-existent PCIe clock setup:

The controller can run at up to 250MHz, so use a parent that provides this
clock.

The PHY needs an exact 100MHz reference clock to function if the PCIe
refclock is not fed in via the refclock pads. While this mode is not
supported (yet) in the driver it doesn't hurt to make sure we are
providing a clock with the right rate.

The AUX clock is specified to have a maximum clock rate of 10MHz. So
the current setup, which drives it straight from the 25MHz oscillator is
actually overclocking the AUX input.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>