smbios: reorganise OEM strings handling.

OEM strings should not be handled by mobo code but by common code with strings
collected from all devices.

Change-Id: Ibde61a1ca79845670bc0df87dc6c67fa868d48a9
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6788
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)