MAINTAINERS: split out the uAPI into a new section

Kent Gibson is the author of the character device uAPI v2 and should be
Cc'ed on all patches aimed for it. Unfortunately this is not the case as
he's not listed in MAINTAINERS. Split the uAPI files into their own
section and make Kent the reviewer.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>