drm/amd/powerplay: add interface to get vbios bootup values (v2)

This patch adds interface to get vbios bootup values for smu.
It's to get boot_values from vbios to set revision, gfxclk, and etc.

v2: add detailed info to describe this function.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>