gpio: Add support for ADI ADP5588 GPIO expander chips

gpio: Add support for ADI ADP5588 GPIO expander chips

This adds support for the ADP588 GPIO expander from Analog Devices. It
is accessed over I2C and provides up to 18 pins. It is largely a port of
the Linux driver developed by Michael Hennerich
<michael.hennerich@analog.com>

Signed-off-by: Ian Roberts <ian.roberts@timesys.com>
Signed-off-by: Greg Malysa <malysagreg@gmail.com>
Signed-off-by: Vasileios Bimpikas <vasileios.bimpikas@analog.com>
Signed-off-by: Utsav Agarwal <utsav.agarwal@analog.com>
Signed-off-by: Arturs Artamonovs <arturs.artamonovs@analog.com>
Signed-off-by: Oliver Gaskell <Oliver.Gaskell@analog.com>
Signed-off-by: Nathan Barrett-Morrison <nathan.morrison@timesys.com>