HSI: hsi: Introducing HSI framework

Adds HSI framework in to the linux kernel.

High Speed Synchronous Serial Interface (HSI) is a
serial interface mainly used for connecting application
engines (APE) with cellular modem engines (CMT) in cellular
handsets.

HSI provides multiplexing for up to 16 logical channels,
low-latency and full duplex communication.

Signed-off-by: Carlos Chinea <carlos.chinea@nokia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>