gpio: Add support for Janz VMOD-TTL Digital IO module

The Janz VMOD-TTL is a MODULbus daughterboard which fits onto any MODULbus
carrier board. It essentially consists of some various logic and a Zilog
Z8536 CIO Counter/Timer and Parallel IO Unit.

The board must be physically configured with jumpers to enable a user to
drive output signals. I am only interested in outputs, so I have made this
driver as simple as possible. It only supports a very minimal subset of the
features provided by the Z8536 chip.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>