media: dt-bindings: Add DW9817 to DW9807 binding

The DW9817 is programmatically the same as DW9807, but
the output drive is a bi-directional -100 to +100mA
instead of 0-100mA.

Add the appropriate compativle string.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>