x86: som-db5800-som-6867: fix SERIRQ on reset

Explicitly enable ILB_SERIRQ function 1 in
cfio_regs_pad_ilb_serirq_PCONF0.

Pad configuration for SERIRQ is not set to enable the SERIRQ function
after a reset though strangely, it is on initial boot.

Rebooting from Linux, reset command in u-boot and even pushing the reset
button on the development board all lead to the SERIRQ function being
disabled (address 0xfed0c560 with value of 0x2003cc80).

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/dts/baytrail_som-db5800-som-6867.dts

@@ -63,6 +63,12 @@
 			pad-offset = <0x270>;
 			mode-func = <2>;
 		};
+
+		/* SERIRQ */
+		soc_gpio_s0_50@0 {
+			pad-offset = <0x560>;
+			mode-func = <1>;
+		};
 	};
 
 	chosen {