Add ns16550a compatible string in UART 8250 driver

Obtained from:         Semihalf
Submitted by:          Michal Stanek <mst@semihalf.com>
Sponsored by:          Annapurna Labs
Approved by:           cognet (mentor)
Reviewed by:           imp, wma
Differential revision: https://reviews.freebsd.org/D5404

sys/dev/uart/uart_dev_ns8250.c

@@ -397,6 +397,7 @@ struct uart_class uart_ns8250_class = {
 #ifdef FDT
 static struct ofw_compat_data compat_data[] = {
 	{"ns16550",		(uintptr_t)&uart_ns8250_class},
+	{"ns16550a",		(uintptr_t)&uart_ns8250_class},
 	{"snps,dw-apb-uart",	(uintptr_t)&uart_ns8250_class},
 	{NULL,			(uintptr_t)NULL},
 };