dt-bindings: i2c: mux-gpio: name correct maintainer

The YAML conversion added me as maintainer but I can't recall being
asked nor do I want to maintain it. Add Peter as maintainer for the
binding as he is maintainer of the driver.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Peter Korsgaard <peter.korsgaard@barco.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>