Input: gpio-beeper - simplify GPIO handling

This patch simplifies GPIO handling in the driver by using GPIO functions
based on descriptors. As a result this driver now can be used for boards
without DT support.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>