gpio: f7188x: use BIT() macro

Align to how we handle bitmasks in most drivers in the
subsystem: using the BIT(n) macro over (1 << n).

Cc: Peter Hung <hpeter@gmail.com>
Cc: Andreas Bofjall <andreas@gazonk.org>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>