semtech has two series of sx150x gpio expanders: sx150x-456 and
sx150x-789.

The current gpio-150x driver in linux only support sx1508 and
sx1509.

We added sx1506 support code into 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>