doc: release-notes-2.2: Update GPIO API section
doc: release-notes-2.2: Update GPIO API section

Add release notes introducing changes to GPIO API.

Signed-off-by: Piotr Mienkowski <be7d9ab625d3cb5e3a4a3202f3529b954dba084c@gmail.com>

nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr