fmc: Decouple from Linux GPIO subsystem

FMC has its own GPIO handling, the inclusion of <linux/gpio.h>
is only to reuse some flags that we can just as well provide
using local defines.

Cc: Federico Vaga <federico.vaga@cern.ch>
Cc: Pat Riehecky <riehecky@fnal.gov>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>