Some encoders have both outputs low in stable states, others also have
a stable state with both outputs high (half-period mode) and some have
a stable state in all steps (quarter-period mode). The driver used to
support the former states and with this change it can also support the
later.

This commit also deprecates the 'half-period' property and introduces
a new property 'steps-per-period'. This property specifies the
number of steps (stable states) produced by the rotary encoder
for each GPIO period.
Signed-off-by: NGuido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: NEzequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>