gpio: rename gpio-langwell to gpio-intel-mid

gpio-langwell is a deprecated name. Despite the driver was made
initially for Langwell, it supports now other Intel Mid SoC's.

This patch does no change beside the file renaming with Kconfig/Makefile
update.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>