mmc: Don't reference Linux-specific OF_GPIO_ACTIVE_LOW flag in DT binding

OF_GPIO_ACTIVE_LOW is a Linux implementation detail. The binding
document should be referring to GPIO_ACTIVE_LOW found in
include/dt-bindings/gpio/gpio.h.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Signed-off-by: Rob Herring <robh@kernel.org>