HSI: Introduce OMAP SSI driver

Add OMAP SSI driver to the HSI subsystem.

The Synchronous Serial Interface (SSI) is a legacy version
of HSI. As in the case of HSI, it is mainly used to connect
Application engines (APE) with cellular modem engines (CMT)
in cellular handsets.

It provides a multichannel, full-duplex, multi-core communication
with no reference clock. The OMAP SSI block is capable of reaching
speeds of 110 Mbit/s.

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>