dts: bindings: add nrf-fll16m and nrf-lfclk bindings

Add bindings for nrf-fll16m and nrf-lfclk and update in-tree nodes
and boards to use them in place of the fixed-clock binding.

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