Input: mpr121 - add polling mode

In case the interrupt line is not available, polling can be used
to read out the state of the keys. Period of the polling needs to
be configured by the poll-interval DT property.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>