Input: adp5588-keys - export unused GPIO pins

This patch allows exporting GPIO pins not used by the keypad itself
to be accessible from elsewhere.

Signed-off-by: Xiaolong Chen <xiao-long.chen@motorola.com>
Signed-off-by: Yuanbo Ye <yuan-bo.ye@motorola.com>
Signed-off-by: Tao Hu <taohu@motorola.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>