ACPI: video: Add Dell UART backlight controller detection

ACPI: video: Add Dell UART backlight controller detection

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

In DSDT this uart port will be defined as:

   Name (_HID, "DELL0501")
   Name (_CID, EisaId ("PNP0501")

Commit 484bae9e4d6a ("platform/x86: Add new Dell UART backlight driver")
has added support for this, but I neglected to tie this into
acpi_video_get_backlight_type().

Now the first AIO has turned up which has not only the DSDT bits for this,
but also an actual controller attached to the UART, yet it is not using
this controller for backlight control.

Add support to acpi_video_get_backlight_type() for a new dell_uart
backlight type. So that the existing infra to override the backlight
control method on the commandline or with DMI quirks can be used.

Fixes: 484bae9e4d6a ("platform/x86: Add new Dell UART backlight driver")
Cc: All applicable <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Rafael J. Wysocki <[email protected]>