Input: add support for the FlySky FS-iA6B RC receiver

Input: add support for the FlySky FS-iA6B RC receiver

This patch adds support for the FlySky FS-iA6B RC receiver (serial IBUS).

It allows the usage of the FlySky FS-i6 and other AFHDS compliant remote
controls as a joystick input device.

To use it, a patch to inputattach which adds the FS-iA6B as a 115200 baud
serial device is required. I will upstream it after this patch is merged.

More information about the hardware can be found here:

https://notsyncing.net/?p=blog&b=2018.linux-fsia6b

Signed-off-by: Markus Koch <markus@notsyncing.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>