tools arch x86: Add dell-uart-backlight-emulator

Dell All In One (AIO) models released after 2017 use a backlight controller
board connected to an UART.

Add a small emulator to allow development and testing of
the drivers/platform/x86/dell/dell-uart-backlight.c driver for
this board, without requiring access to an actual Dell All In One.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240513144603.93874-3-hdegoede@redhat.com