drm/i2c: tda998x: fix lack of required reg in DT documentation

The I2C address (reg) is required for the TDA998x driver to be loaded
and initialized.
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Documentation/devicetree/bindings/drm/i2c/tda998x.txt

@@ -3,6 +3,8 @@ Device-Tree bindings for the NXP TDA998x HDMI transmitter
 Required properties;
   - compatible: must be "nxp,tda998x"
 
+  - reg: I2C address
+
 Optional properties:
   - interrupts: interrupt number and trigger type
 	default: polling