char: i.MX: Add support for "TX complete" interrupt

char: i.MX: Add support for "TX complete" interrupt

Add support for "TX complete"/TXDC interrupt generate by real HW since
it is needed to support guests other than Linux.

Based on the patch by Bill Paul as found here:
https://bugs.launchpad.net/qemu/+bug/1753314

Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: Bill Paul <wpaul@windriver.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Bill Paul <wpaul@windriver.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Message-id: 20180315191141.6789-2-andrew.smirnov@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>