backlight: aw99706: Add support for Awinic AW99706 backlight

backlight: aw99706: Add support for Awinic AW99706 backlight

Add support for Awinic AW99706 backlight, which can be found in
tablet and notebook backlight, one case is the Lenovo Legion Y700
Gen4. This driver refers to the official datasheets and android
driver, they can be found in [1].

[1] https://www.awinic.com/en/productDetail/AW99706QNR

Signed-off-by: Junjie Cao <caojunjie650@gmail.com>
Tested-by: Pengyu Luo <mitltlatltl@gmail.com>
Signed-off-by: Pengyu Luo <mitltlatltl@gmail.com>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Link: https://patch.msgid.link/20251109032240.3422503-3-caojunjie650@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>