gpio: intel-mid: Make it depend to X86_INTEL_MID

This GPIO controller is a part of Intel MID platforms which are somehow
different to pure PCs. Thus, there is no need that driver is compiled for them.

Replace dependency to X86_INTEL_MID.

While here, fix capitalization of MID abbreviation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>