i.MX: Fix GPIO ISR register write

Writing the ISR register is supposed to clear interrupt status bits,
not to set them.

This patch makes '-M sabrelite' work without devicetree changes (Linux
kernel versions 3.18 to 4.7 with imx_v6_v7_defconfig and up to v4.8 with
multi_v7_defconfig; mainline has different problems).

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 1477361005-18646-1-git-send-email-linux@roeck-us.net
Acked-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>