Input: adp5589 - make keypad support optional

On some platforms the adp5589 is used in GPIO only mode. On these platforms
we do not want to register a input device, so make that optional and only
create the input device if a keymap is supplied.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20191023070541.13940-1-alexandru.ardelean@analog.com


[dtor: dropped unnecessary changes related to passing pdata to various
 functions]
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>