syscon: dm: Add a new method to get a regmap from DTS

syscon_regmap_lookup_by_phandle() can be used to get the regmap of a syscon
device from a reference in the DTS. It operates similarly to the linux
version of the namesake function.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/syscon.c b/test/dm/syscon.c
index 77c7928..a294dda 100644
--- a/test/dm/syscon.c
+++ b/test/dm/syscon.c
@@ -6,6 +6,7 @@
 #include <common.h>
 #include <dm.h>
 #include <syscon.h>
+#include <regmap.h>
 #include <asm/test.h>
 #include <dm/test.h>
 #include <test/ut.h>
@@ -43,3 +44,31 @@
 	return 0;
 }
 DM_TEST(dm_test_syscon_by_driver_data, DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT);
+
+/* Test system controller by phandle */
+static int dm_test_syscon_by_phandle(struct unit_test_state *uts)
+{
+	struct udevice *dev;
+	struct regmap *map;
+
+	ut_assertok(uclass_get_device_by_name(UCLASS_TEST_PROBE, "test4",
+					      &dev));
+
+	ut_assertok_ptr(syscon_regmap_lookup_by_phandle(dev, "first-syscon"));
+	map = syscon_regmap_lookup_by_phandle(dev, "first-syscon");
+	ut_assert(map);
+	ut_assert(!IS_ERR(map));
+	ut_asserteq(1, map->range_count);
+
+	ut_assertok_ptr(syscon_regmap_lookup_by_phandle(dev,
+							"second-sys-ctrl"));
+	map = syscon_regmap_lookup_by_phandle(dev, "second-sys-ctrl");
+	ut_assert(map);
+	ut_assert(!IS_ERR(map));
+	ut_asserteq(4, map->range_count);
+
+	ut_assert(IS_ERR(syscon_regmap_lookup_by_phandle(dev, "not-present")));
+
+	return 0;
+}
+DM_TEST(dm_test_syscon_by_phandle, DM_TESTF_SCAN_PDATA | DM_TESTF_SCAN_FDT);