gpio: mrvl: dt-binding: remove orion-gpio description

The Orion GPIO controller binding description in mrvl-gpio.txt is obsolete, and
duplicates the description in gpio-mvebu.txt.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>