kboot: Powerpc provide bi_loadsmap

It's just a stub, since the kernel learns of memory via FDT.

Sponsored by:		Netflix
Reviewed by:		tsoome, kevans
Differential Revision:	https://reviews.freebsd.org/D38253

stand/kboot/arch/powerpc64/load_addr.c

@@ -220,3 +220,9 @@ bool enumerate_memory_arch(void)
 
 	return true;
 }
+
+void
+bi_loadsmap(struct preloaded_file *kfp)
+{
+	/* passed in via the DTB */
+}