dm: i2c: Add functions to read and write a register

Add driver model versions of the legacy functions to read and write a
single byte register. These are a useful shortcut in many cases.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
diff --git a/drivers/i2c/i2c-uclass.c b/drivers/i2c/i2c-uclass.c
index f2e95c0..b8eb2d6 100644
--- a/drivers/i2c/i2c-uclass.c
+++ b/drivers/i2c/i2c-uclass.c
@@ -186,6 +186,25 @@
 	}
 }
 
+int dm_i2c_reg_read(struct udevice *dev, uint offset)
+{
+	uint8_t val;
+	int ret;
+
+	ret = dm_i2c_read(dev, offset, &val, 1);
+	if (ret < 0)
+		return ret;
+
+	return val;
+}
+
+int dm_i2c_reg_write(struct udevice *dev, uint offset, uint value)
+{
+	uint8_t val = value;
+
+	return dm_i2c_write(dev, offset, &val, 1);
+}
+
 /**
  * i2c_probe_chip() - probe for a chip on a bus
  *