RISC-V: build SiFive drivers and DTB in GENERIC

In the spirit of the GENERIC config, we should include the drivers required to
run on most supported platforms.

Reviewed by:	kp
Differential Revision:	https://reviews.freebsd.org/D26501

sys/riscv/conf/GENERIC

@@ -163,4 +163,10 @@ device		firmware	# firmware assist module
 # Note that 'bpf' is required for DHCP.
 device		bpf		# Berkeley packet filter
 
+# Flattened Device Tree
 options 	FDT
+makeoptions	MODULES_EXTRA+="dtb/sifive"
+
+# SiFive device drivers
+device		fu540spi
+include		"../sifive/std.sifive"