Add compatibility string for dw-apb-uart in ns8250 driver

This compatibility string is used in .dts file of Armada38x
and isrequired for driver attachment.

Reviewed by:    andrew, ian, imp
Obtained from:  Semihalf
Sponsored by:   Stormshield
Submitted by:   Michal Stanek <mst@semihalf.com>
Differential revision:  https://reviews.freebsd.org/D4216

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},
+	{"snps,dw-apb-uart",	(uintptr_t)&uart_ns8250_class},
 	{NULL,			(uintptr_t)NULL},
 };
 UART_FDT_CLASS_AND_DEVICE(compat_data);