greybus: bundle: add state sysfs file

A bundle has a state file, that is managed by the endo userspace
process.  This file can be written to and any process that is polling on
the file will be woken up and can read the new value.  It's a "cheap"
IPC for programs that are not allowed to do anything other than
read/write to kernel sysfs files.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>