gpio: syscon: Change the compatibility string

This patch changes the compatibility string to match with the smallest
supported chip (EP7209). Since the DT-support for this CPU is not yet
announced, this change is safe.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>