gpiodev: Add GPIO device frontend

gpiodev: Add GPIO device frontend

Add GPIO device front end with helper functions to provide information
about GPIO Port to Backends and to allow Frontend to set data.

To use it GPIO device should register with qemu_gpio_fe_init()
and provide handlers with qemu_gpio_fe_set_handlers().

Notifications about config and changed value events should be done with
qemu_gpio_fe_line_event() and qemu_gpio_fe_config_event().

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>