hw/char/exynos4210_uart: Implement Rx FIFO level triggers and timeouts

hw/char/exynos4210_uart: Implement Rx FIFO level triggers and timeouts

The driver already implements a receive FIFO, but it does not
handle receive FIFO trigger levels and timeout. Implement the
missing functionality.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20200123052540.6132-7-linux@roeck-us.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>