dtoverlays: Add support for ADV7280-M, ADV7281-M and ADV7281-MA chips.

The driver that supports the ADV7282-M also supports the ADV7280-M,
ADV7281-M, and ADV7281-MA.
The 7280-M exposes 8 analogue inputs. The 7281-M doesn't have the
I2P deinterlacing block. The 7281-MA has 8 inputs but no I2P.
Otherwise they are the same as ADV7282-M.

Adds a new overlay "adv728x" that includes the existing adv7282
overlay but adds several parameters to modify the behaviour.

Adds a new addr parameter to allow the I2C address to be changed.
(the chip has an address select pin to change between 0x20 and 0x21).

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