gpio/Kconfig: Fix X86 arch name

X86 Kconfig symbol is X86, not ARCH_X86.

Fixes: c586b3075d5b47d8 (gpio/xilinx: Add support for X86 Arch)
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>