gpio: sx150x: add dts support for sx150x driver

Current sx150x gpio expander driver doesn't support
DTS. Now we added dts support for this driver.

Signed-off-by: Wei Chen <Wei.Chen@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>