dts: bindings: clock: rename nordic,nrf-lfxo -> nordic,nrf54l-lfxo

Each nrf series has its own variant of the lfxo clock, specified
by the binding which includes the series in the name. The nrf54L
series is no different, hence the binding should be clearly
specified by having the series name within it.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>