greybus: uart: Add gb_uart_request_recv for receiving async UART data

gb_uart_request_recv job in life is to process unsolicited greybus
mesages from the UART.
Hook the incoming UART data and pass to the TTY layer.
Line-state changes still TBD.

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