dm: core: Add a way to look up a phandle in an oftree

When we have multiple trees, the ofnode logic needs to be told which one
to use. Create a new function which takes an oftree argument, along with
a helper to obtain the FDT pointer from an oftree.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/ofnode.c

@@ -36,6 +36,8 @@ static int dm_test_ofnode_get_by_phandle(struct unit_test_state *uts)
 	/* test unknown phandle */
 	ut_assert(!ofnode_valid(ofnode_get_by_phandle(0x1000000)));
 
+	ut_assert(ofnode_valid(oftree_get_by_phandle(oftree_default(), 1)));
+
 	return 0;
 }
 DM_TEST(dm_test_ofnode_get_by_phandle, UT_TESTF_SCAN_PDATA | UT_TESTF_SCAN_FDT);