This patch adds GPIO support for CP2102N devices.

It introduces new generic code to support emulating separate
input and outputs directions even though these devices
only know output modes (open-drain and pushpull). Existing
GPIO support for CP2105 has been migrated over to the new
code structure.

Only limitation is that for the QFN28 variant, only 4 out of
7 GPIOs are supported. This is because the config array
locations of the last 3 pins are not documented, and reverse
engineering revealed offsets that conflicted with other
documented functions. Hence we'll play it safe instead
until somebody clears this up further.
Signed-off-by: Karoly Pados <pados@pados.hu>
[ johan: fix style issues and a couple of minor bugs; use Karoly's
         updated commit message ]
Acked-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>