firmware: dmi: Add macro SMBIOS_ENTRY_POINT_SCAN_START

Use SMBIOS_ENTRY_POINT_SCAN_START instead of 0xF0000, because other
archtecture maybe use a special start address such as 0xFFFE000 for
Loongson platform.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>