gpio: group port-mapped I/O drivers in a menu

Create a Kconfig submenu for drivers using X86 port-mapped I/O
and depend on X86 for this.

Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: Andreas Bofjall <andreas@gazonk.org>
Cc: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Cc: Daniel Krueger <daniel.krueger@systec-electronic.com>
Cc: Bruno Randolf <br1@einfach.org>
Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>