gpio: sx150x: add Semtech I2C sx150x gpio expander driver

Add support for Semtech SX150-series I2C GPIO expanders.  Compatible
models include:

8 bits:  sx1508q
16 bits: sx1509q

Signed-off-by: Gregory Bean <gbean@codeaurora.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>