gpio: tangier: calculate number of ctx using temporary variable

Utilize a temporary variable to calculate number of ctx from ngpio
inside ->probe() implementation.
While at it, include math.h for using DIV_ROUND_UP().

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>