silabs: siwx917: Fix UART initialization

Fix LPUART initialization to match what wiseconnect uses.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

Author: jhedberg

dts/arm/silabs/siwg917.dtsi

@@ -45,7 +45,7 @@
 			reg = <0x24041800 0x1000>;
 			interrupts = <12 0>;
 			reg-shift = <2>;
-			clock-frequency = <15000000>;
+			clock-frequency = <13000000>;
 			current-speed = <115200>;
 			status = "disabled";
 		};

soc/silabs/silabs_siwx917/siwg917/soc.c

@@ -16,7 +16,8 @@ int silabs_siwx917_init(void)
 
 	/* FIXME: do not hardcode UART instances */
 #if DT_NODE_HAS_STATUS(DT_NODELABEL(ulpuart0), okay)
-	RSI_ULPSS_UlpUartClkConfig(ULPCLK, ENABLE_STATIC_CLK, 0, ULP_UART_ULP_32MHZ_RC_CLK, 1);
+	RSI_PS_UlpssPeriPowerUp(ULPSS_PWRGATE_ULP_UART);
+	RSI_ULPSS_UlpUartClkConfig(ULPCLK, ENABLE_STATIC_CLK, 0, ULP_UART_REF_CLK, 0);
 #endif
 #if DT_NODE_HAS_STATUS(DT_NODELABEL(uart0), okay)
 	RSI_CLK_UsartClkConfig(M4CLK, ENABLE_STATIC_CLK, 0, USART1, 0, 1);