drivers: serial: Implement half-duplex mode for nordic,nrf-uarte serial driver interrupt driven mode

[cormoran]

This pull request implements half-duplex mode for nRF serial driver to use wired split transport with single line for nRF devices.
The implementation is a bit hacky and I'm not sure you want to merge this change directly to this fork, but I expect it helps people who is willing to experiment 1-wire wired transport with nRF devices.

The implementation works only for interrupt driven mode for simplicity.
I tested the implementation with my XIAO nRF52840 plus custom keyboard with following setup.
And you should implement protection from collision in your circuit like inserting resistance in the connection line. Otherwise, the chip might be damaged at unexpected TX collision.

# your_keyboard.conf
CONFIG_ZMK_SPLIT_WIRED_UART_MODE_INTERRUPT=y
# And be sure your uart is configured as interrupt driven mode
# CONFIG_UART_0_INTERRUPT_DRIVEN=y

# your_keyboard.dtsi
/ {
    wired_split {
        compatible = "zmk,wired-split";
        device = <&uart0>;
        half-duplex;
    };
};
&uart0 {
    compatible = "nordic,nrf-uarte";
    current-speed = <19200>;
    status = "okay";
    pinctrl-0 = <&uart0_rx>;
    pinctrl-1 = <&uart0_sleep>;
    pinctrl-2 = <&uart0_tx>;
    pinctrl-names = "rx", "sleep", "tx";
    half-duplex;
};

// example of using P0.03 (D1) as uart pin
&pinctrl {
    uart0_rx: uart0_rx {
        group1 {
            psels = <NRF_PSEL(UART_RX, 0, 3)>;
            bias-pull-up;
        };
        group2 {
            psels = <NRF_PSEL_DISCONNECTED(UART_TX)>;
        };
    };
    uart0_sleep: uart0_sleep {
        group1 {
            psels = <NRF_PSEL(UART_RX, 0, 3)>, <NRF_PSEL_DISCONNECTED(UART_TX)>;
            low-power-enable;
        };
    };
    uart0_tx: uart0_tx {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 3)>;
            bias-pull-up;
        };
        group2 {
            psels = <NRF_PSEL_DISCONNECTED(UART_RX)>;
        };
    };
};