[coreboot] Intel GMA: Backlight brightness driver

[Patrick Rudolph]

Dear coreboot users,
The following commit adds Mailbox 3 support for improved panel back-
light brigthness control on Intel GMA platforms.

It fixes the problem with 100% brightness on S3 resume.

Please test and review https://review.coreboot.org/#/c/coreboot/+/27711
/ 

Regards,
Patrick