platform/x86: Add new Dell UART backlight driver

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")

Instead of having a separate ACPI device with an UartSerialBusV2() resource
to model the backlight-controller, which would be the standard way to do
this.

The acpi_quirk_skip_serdev_enumeration() has special handling for this
and it will make the serial port code create a serdev controller device
for the UART instead of a /dev/ttyS0 char-dev. It will also create
a dell-uart-backlight driver platform device for this driver to bind too.

This new kernel module contains 2 drivers for this:

1. A simple platform driver which creates the actual serdev device
   (with the serdev controller device as parent)

2. A serdev driver for the created serdev device which exports
   the backlight functionality uses a standard backlight class device.

Reported-by: Roman Bogoyev <[email protected]>
Tested-by: Roman Bogoyev <[email protected]>
Tested-by: Kai-Heng Feng <[email protected]>
Co-developed-by: AceLan Kao <[email protected]>
Signed-off-by: AceLan Kao <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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