smbios: get_str_from_dt() - add sysinfo_id description

Add description for parameter sysinfo_id of function get_str_from_dt().

Fixes: 07c9e683 ("smbios: Allow a few values to come from sysinfo")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>