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>