greybus: firmware: Add SPI protocol support

This patch adds SPI Protocol support to firmware core, which allows the
AP to access an SPI flash memory present with an Interface.

Tested by using the API from fw-management driver and compiling it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>