hwmon: (pmbus/adm1266) Add support for GPIOs

Adm1266 exposes 9 GPIOs and 16 PDIOs which are currently read-only. They
are controlled by the internal sequencing engine.

This patch makes adm1266 driver expose GPIOs and PDIOs to user-space
using GPIO provider kernel api.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Link: https://lore.kernel.org/r/20200812142055.9213-4-alexandru.tachici@analog.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>