platform/x86: x86-android-tablets: Create LED device for Xiaomi Pad 2 bottom bezel touch buttons

The Xiaomi [Mi]Pad 2 has 3 menu / home / back capacitive touch-buttons
on its bottom bezel. These are backlit by LEDs attached to a TPS61158 LED
controller which is controlled by the "pwm_soc_lpss_2" PWM output.

Create a LED class device for this, using the new input-events trigger
as default trigger so that the buttons automatically light up on any
input activity.

Note alternatively a "leds_pwm" platform device could be created together
with the necessary fwnode_s_ and a fwnode link to the PWM controller.
There are 2 downsides to this approach:

1. The code would still need to pwm_get() the PWM controller to get/attach
a fwnode for the PWM controller fwnode link and setting up the necessary
fwnodes is non-trivial. So this would likely require more code then simply
registering the LED class device directly.

2. Currently the leds_pwm driver and its devicetree bindings do not support
limiting the maximum dutycycle to less then 100% which is required in this
case (the leds_pwm driver can probably be extended to allow this).

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240509141207.63570-2-hdegoede@redhat.com