platform/x86: x86-android-tablets: Add swnode for Xiaomi pad2 indicator LED

There is a KTD2026 LED controller to manage the indicator LED for Xiaomi
pad2. The ACPI for it is not properly made so the kernel can't get
a correct description of it.

This work adds a description for this RGB LED controller and also sets
a trigger to indicate the charging event (bq27520-0-charging). When it
is charging, the indicator LED will be turned on.

Signed-off-by: Kate Hsuan <hpa@redhat.com>
Link: https://lore.kernel.org/r/20240322033736.9344-2-hpa@redhat.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>