smbios: Allow a few values to come from sysinfo

While static configuration is useful it cannot cover every case. Sometimes
board revisions are encoded in resistor straps and must be read at
runtime.

The easiest way to provide this information is via sysinfo, since the
board can then provide a driver to read whatever is needed.

Add some standard sysinfo options for this, and use them to obtain the
required information.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>