How to put the UART to sleep (low power mode)

I believe it started with SDK 2.0.0 but the board config files now define different ways to specify how hardware is to run.

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&pinctrl {
    uart1_default: uart1_default {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 26)>,
                <NRF_PSEL(UART_RTS, 0, 29)>;
        };
        group2 {
            psels = <NRF_PSEL(UART_RX, 0, 27)>,
                <NRF_PSEL(UART_CTS, 0, 28)>;
            bias-pull-up;
        };
    };
    uart1_sleep: uart1_sleep {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 26)>,
                <NRF_PSEL(UART_RX, 0, 27)>,
                <NRF_PSEL(UART_RTS, 0, 29)>,
                <NRF_PSEL(UART_CTS, 0, 28)>;
            low-power-enable;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

&pinctrl {

    uart1_default: uart1_default {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 26)>,
				<NRF_PSEL(UART_RTS, 0, 29)>;
		};
		group2 {
			psels = <NRF_PSEL(UART_RX, 0, 27)>,
				<NRF_PSEL(UART_CTS, 0, 28)>;
			bias-pull-up;
		};
	};

	uart1_sleep: uart1_sleep {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 26)>,
				<NRF_PSEL(UART_RX, 0, 27)>,
				<NRF_PSEL(UART_RTS, 0, 29)>,
				<NRF_PSEL(UART_CTS, 0, 28)>;
			low-power-enable;
		};
	};
	
&uart1 {
	status = "okay";
	pinctrl-0 = <&uart1_default_alt>;
	pinctrl-1 = <&uart1_sleep_alt>;
	pinctrl-names = "default", "sleep";
};

I took this to mean that I can dynamically toggle these configurations from "uart1_default" to "uart1_sleep" from perhaps a function call. Did find the pinctrl_apply_state() function call but it's return a -2 that state id doesn't exist even though it is defined. Here's my code.

uart = device_get_binding("UART_1");

int err = pinctrl_apply_state(uart->config, PINCTRL_STATE_SLEEP); // Returns -2

Any ideas?