greybus: uart: Move UART protocol structs/defines to greybus_protocols.h

gbsim depends on the structures and defines in greybus_protocols.h
generally in order to simulate firmware. Move UART defines into this
header to facilitate.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>