gpio: Port SX150x driver from Linux ...

Add a very abridged version of SX150x driver from Linux. New, "pinctrl"
version of the driver was used as a base. As it was already mentioned
this driver supports very limited amount of the original functionality,
and the following are the features that were dropped:
     - Interrupt support
     - Support for any chip other that SX150x (due to lack of HW to test
       with)
     - Any pinctlr-like functions: pull-up/pull-down, open-drain,
       etc. configuration

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>