HSI: Introduce driver for SSI Protocol

This adds a driver for the SSI McSAAB protocol as used in
the Nokia N900.

Signed-off-by: Carlos Chinea <carlos.chinea@nokia.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Tested-By: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>