drm/amdgpu: Enable reading FRU chip via I2C v3

Allow for reading of information like manufacturer, product number
and serial number from the FRU chip. Report the serial number as
the new sysfs file serial_number. Note that this only works on
server cards, as consumer cards do not feature the FRU chip, which
contains this information.

v2: Add documentation to amdgpu.rst, add helper functions,
    rename functions for consistency, fix bad starting offset
v3: Remove testing definitions

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>