greybus: Greybus UART connection driver

Flush out the Greybus UART driver to actually implement greybus
requests.  The number of Greybus Protocol operations has been reduced
down to a managable number, and, if you look closely, you will notice it
follows the CDC ACM USB specification, which can drive UART devices
quite well, no need for complex UART state changes, leave all of that
logic up to the firmware, if it wants/needs it.

The Greybus Protocol spec has been updated to match the driver.

TODO: There are 2 requests from the device to the host that need to be
implemented.  As this isn't fully hooked up in the Greybus core, that is
not implemented here yet either.
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>