i2c: Implement i2c_get_chip_by_phandle()

This new function enhances the i2c_get_chip*() toolbox by implementing a
variant that does not require a chip_addr. Instead, the desired device
is pointed to by a phandle.

Signed-off-by: Philip Richard Oberfichtner <pro@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/include/i2c.h b/include/i2c.h
index ef3820e..4e59009 100644
--- a/include/i2c.h
+++ b/include/i2c.h
@@ -538,6 +538,18 @@
 			    struct udevice **devp);
 
 /**
+ * i2c_get_chip_by_phandle() - get a device to use to access a chip
+ *			       based on a phandle property pointing to it
+ *
+ * @parent: Parent device containing the phandle pointer
+ * @name:   Name of phandle property in the parent device node
+ * @devp:   Returns pointer to new device or NULL if not found
+ * Return:  0 on success, -ve on failure
+ */
+int i2c_get_chip_by_phandle(const struct udevice *parent, const char *prop_name,
+			    struct udevice **devp);
+
+/**
  * i2c_chip_of_to_plat() - Decode standard I2C platform data
  *
  * This decodes the chip address from a device tree node and puts it into