drivers: pinctrl: nrf: add support for disconnecting a pin

It was not possible to disconnect a pin using the nRF pinctrl driver.
That is, it was not possible to set PSEL to 0xFFFFFFFF (indicating pin
is not connected). This can be useful in certain scenarios, e.g. a
bootloader configures all signals of a certain peripheral but
application then needs to disconnect certain signals.

A new DT macro has been introduced to accomplish this:
NRF_PSEL_DISCONNECT. It can be used like this to explicitely disconnect
a peripheral signal:

```
&pinctrl {
	uart0_default: uart0_default {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 1)>,
				<NRF_PSEL_DISCONNECTED(UART_RX)>;
		};
	};
};
```

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>