ARM: EXYNOS: Update CONFIG_ARCH_NR_GPIO for Exynos

With the recent cleanup in Exynos platform code notably commits
17859bec ("ARM: EXYNOS: Do not select legacy Kconfig symbols any
more") and b9222210 ("ARM: EXYNOS: Remove mach/gpio.h"), the definition
of ARCH_NR_GPIOS got removed. This started causing problems on SoCs like
Exynos4412 which have more than the default number of GPIOs. Thus define
this number in KConfig file which takes care of current SoC requirements
and provides scope for GPIO expanders. Without this patch we get the
following errors during boot:

gpiochip_add: gpios 251..258 (gpv0) failed to register
samsung-pinctrl 106e0000.pinctrl: failed to register gpio_chip gpv0, error code: -22
samsung-pinctrl: probe of 106e0000.pinctrl failed with error -22

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>