ARM: dts: ux500: use the GPIO DT header

Use the <dt-bindings/gpio/gpio.h> header instead of using
hardcoded values for the GPIO flags. Eradicate the totally
bogus "0x4" flag used and set that to GPIO_ACTIVE_HIGH as is
proper, switch the inverted card detect on the Snowball to
flag using GPIO_ACTIVE_LOW instead of using the MMC-specific
inversion flag.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>