dt-bindings: clock: meson8b: remove non-existing clock macros

CLKID_UNUSED and CLKID_XTAL aren't valid clocks. Remove them since
there are no consumers of this anymore.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20201221183624.932649-3-martin.blumenstingl@googlemail.com