applications: serial_lte_modem: Fix HWFC problems on nRF9151DK

nRF9151DK has interface chip that automatically detects if flow
control is enabled or not. On start up, HWFC pins are floating.

On Zephyr DTS side, nRF9151 CTS pin has pull-up enabled, so if
nRF91 app core boots faster than interface chip, the serial port
might be blocked because of flow control.

Change pull-up to pull-down so nRF91 is allowed to write if interface
chip still has not enabled the HWFC pins.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

applications/serial_lte_modem/boards/nrf9151dk_nrf9151_ns.overlay

@@ -13,6 +13,9 @@
 &uart0 {
 	status = "okay";
 	hw-flow-control;
+	pinctrl-0 = <&uart0_default_hwfc>;
+	pinctrl-1 = <&uart0_sleep>;
+	pinctrl-names = "default", "sleep";
 };
 
 &uart2 {
@@ -31,6 +34,20 @@
 };
 
 &pinctrl {
+	uart0_default_hwfc: uart0_default_hwfc {
+		group1 {
+			psels = <NRF_PSEL(UART_TX, 0, 27)>,
+				<NRF_PSEL(UART_RTS, 0, 14)>;
+		};
+		group2 {
+			psels = <NRF_PSEL(UART_RX, 0, 26)>;
+			bias-pull-up;
+		};
+		group3 {
+			psels=	<NRF_PSEL(UART_CTS, 0, 15)>;
+			bias-pull-down;
+		};
+	};
 	uart2_default_alt: uart2_default_alt {
 		group1 {
 			psels = <NRF_PSEL(UART_RX, 0, 11)>;