MAINTAINERS: Do maintain Intel GPIO drivers via separate tree

We would like to consolidate Intel pure GPIO drivers, including PMICs and
some old x86 platforms, in one tree which is maintained by Intel.

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