riscv: build allwinner DTBs

These are known to work if loaded manually by loader(8) (for the Nezha
board at least). If nothing else, it is useful to provide a DTB closely
tied to the kernel version.

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D53118

sys/modules/dtb/allwinner/Makefile

@@ -65,7 +65,12 @@ DTSO=	sun50i-a64-mmc0-disable.dtso \
 	sun50i-a64-timer.dtso \
 	sun50i-h5-opp.dtso \
 	sun50i-h5-nanopi-neo2-opp.dtso
-
+.elif ${MACHINE_CPUARCH} == "riscv"
+DTS=	\
+	allwinner/sun20i-d1-dongshan-nezha-stu.dts \
+	allwinner/sun20i-d1-lichee-rv.dts \
+	allwinner/sun20i-d1-mangopi-mq-pro.dts \
+	allwinner/sun20i-d1-nezha.dts
 .endif
 
 .include <bsd.dtb.mk>

sys/riscv/conf/std.allwinner

@@ -17,4 +17,7 @@ device		awg		# Allwinner EMAC Gigabit Ethernet
 
 device		musb		# Mentor Graphics USB OTG controller
 
+# DTBs
+makeoptions	MODULES_EXTRA+="dtb/allwinner"
+
 files		"../allwinner/files.allwinner"