gpio: Constify platform_device_id

The platform_device_id is not modified by the driver and core uses it as
const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

diff --cc drivers/gpio/gpio-mxc.c
Simple merge

diff --cc drivers/gpio/gpio-mxs.c
Simple merge

diff --cc drivers/gpio/gpio-ts5500.c
Simple merge