gpio: mvebu: fix warning when building on 64-bit

Casting a pointer to an int is not portable, and provokes a compiler
warning.  Cast to unsigned long instead to avoid the warning.

drivers/gpio/gpio-mvebu.c: In function 'mvebu_gpio_probe':
drivers/gpio/gpio-mvebu.c:662:17: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   soc_variant = (int) match->data;
                 ^

This will be needed when building gpio-mvebu for Armada 7k/8k ARM64
SoCs.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>