dt-bindings: tda998x: add the calibration gpio

Add the optional calibration gpio for integrated TDA9950 CEC support.
This GPIO corresponds with the interrupt from the TDA998x, as the
calibration requires driving the interrupt pin low.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>