drivers: pinctrl_nrf: Set clockpin bitfield when necessary

For certain peripheral signal lines in nRF54H20, it is required
to set the clockpin bitfield for pins assigned to them, otherwise
the peripheral may not work properly, for example, there will be
no output from UART.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>