platform: arm64: thinkpad-t14s-ec: new driver

platform: arm64: thinkpad-t14s-ec: new driver

Introduce EC driver for the ThinkPad T14s Gen6 Snapdragon, which is in
theory compatible with ThinkPad ACPI. On Linux the system is booted with
device tree, which is not supported by the ThinkPad ACPI driver
(drivers/platform/x86/lenovo/thinkpad_acpi.c). Also most of the hardware
compatibility is handled via ACPI tables, which are obviously not used
when booting via device tree. Thus adding DT compatibility to the
existing driver is not worth it as there is almost no code sharing.

The driver currently exposes features, which are not available
via other means:

 * Extra Keys
 * System LEDs
 * Keyboard Backlight Control

The driver has been developed by reading the ACPI DSDT. There
are some more features around thermal control, which are not
yet supported by the driver.

The speaker mute and mic mute LEDs need some additional changes
in the ALSA UCM to be set automatically.

Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on Thinkpad T14S OLED
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Link: https://patch.msgid.link/20250918-thinkpad-t14s-ec-v5-2-ac0bc6382c5c@collabora.com
[ij: folded in patch from Chen Ni <nichen@iscas.ac.cn>]
Link: https://patch.msgid.link/20250926091302.817919-1-nichen@iscas.ac.cn
[ij: folded in patch from Lukas Bulwahn <lbulwahn@redhat.com>]
Link: https://patch.msgid.link/20250926071859.138396-1-lukas.bulwahn@redhat.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>