It turns out that not all H6 boards have external 32kHz oscillator.
Currently the only one known such H6 board is Tanix TX6.

Move external oscillator node from common H6 dtsi to board specific dts
files where present.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>