clk: meson: meson8b: don't register the XTAL clock when provided via OF

The XTAL clock is an actual crystal on the PCB. Thus the meson8b clock
driver should not register the XTAL clock - instead it should be
provided via .dts and then passed to the clock controller.

Skip the registration of the XTAL clock if a parent clock is provided
via OF. Fall back to registering the XTAL clock if this is not the case
to keep support for old .dtbs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>