Input: rotary-encoder - support more than 2 gpios as input

This changes how the used gpios are stored (i.e. a struct gpio_descs
instead of two struct gpio_desc) and as with >2 gpios the states are
numbered differently the function rotary_encoder_get_state returns
unencoded numbers instead of grey encoded numbers before. The latter has
some implications on how the returned value is used and so the change is
bigger than one might expect at first.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>