soc/amd/common: add rudimentary ATIF support

soc/amd/common: add rudimentary ATIF support

The Linux kenerl driver for AMD gpu currently has a floor
value of 12 for brightness settings (AMDGPU_DM_DEFAULT_MIN_BACKLIGHT).
AMD indicates they did this because they were concerned with certain
panels flickering at lower backlight values. However, for unaffected
panels it's desirable to be able to have the panel "turn off" at
the lowest backlight setting. The only way to do that is to provide
ATIF bindings that indicate backlight range.

Option SOC_AMD_COMMON_BLOCK_GRAPHICS_ATIF is added to provide a full
range for the backlight setting. If needed, this path can be built upon
for fuller support, but for the time being this is the only thing
necessary to make the backlight be full range.

BUG=b:163583825

Change-Id: If76801a8daf6a5e56ba7d118956f3ebce74e567a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44642
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>