dts: common: nordic: adjust comparator nodes

Adjust comparator nodes of nrf SoCs to exclude the unused
io-channel-cells property and simplify the comment describing how
to configure the comparator hardware block as COMP or LPCOMP for
SoCs which support this.

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