greybus: uart: Add credits based tracking for transmit path

To avoid polling the firmware for space, a credit based system
is implemented.

The host will keep track of how many credits (bytes) it has
sent to the firmware, and stop sending data when the quota
is filled.

The host will be informed that the firmware has more
room for data when it handles the receive_credits request
message from the firmware, and will continue to write data
as credits become available.

The firmware and the host may implement an algorithm to aggregate
credits, and avoid extra greybus traffic.

Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>