gpio: Add driver for 74x164 compatible shift-registers ...

A 74x164 shift register can be seen as a SPI attached GPIO expander.
This adds a driver for those poor-man expanders based on the Linux
driver.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>