platform: arm64: thinkpad-t14s-ec: add suspend handler for keyboard backlight

platform: arm64: thinkpad-t14s-ec: add suspend handler for keyboard backlight

This ensures that the keyboard backlight state is restored after
a suspend-resume cycle. Otherwise the keyboard is automatically
disabled during suspend and then stays disabled after resume.

Note, that this adopts the same behavior as the existing Thinkpad
ACPI driver.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Link: https://patch.msgid.link/20251119-thinkpad-t14s-ec-improvements-v2-4-441219857c02@kernel.org
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>