greybus: endo: add endo structures and logic

This adds endo.c and endo.h and provides functions to create an endo and
the initial 0x0555 set of modules.

But, it doesn't hook this logic up into the running code yet, that comes
next.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>