greybus: spi: Move structure definitions into gpbridge.h

In order to facilitate re-use of spi structures, split them out of
independent files and add them into a shared gpbridge.h

This will be a prereq to sharing these headers w/ gbsim.

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