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/include/syscon.h b/include/syscon.h
index 2aa73e5..3df96e3 100644
--- a/include/syscon.h
+++ b/include/syscon.h
@@ -74,6 +74,19 @@
 struct regmap *syscon_get_regmap_by_driver_data(ulong driver_data);
 
 /**
+ * syscon_regmap_lookup_by_phandle() - Look up a controller by a phandle
+ *
+ * This operates by looking up the given name in the device (device
+ * tree property) of the device using the system controller.
+ *
+ * @dev:	Device using the system controller
+ * @name:	Name of property referring to the system controller
+ * @return	A pointer to the regmap if found, ERR_PTR(-ve) on error
+ */
+struct regmap *syscon_regmap_lookup_by_phandle(struct udevice *dev,
+					       const char *name);
+
+/**
  * syscon_get_first_range() - get the first memory range from a syscon regmap
  *
  * @driver_data:	Driver data value to look up