mtd: ubi: attach from device tree

Introduce device tree compatible 'linux,ubi' and attach compatible MTD
devices using the MTD add notifier. This is needed for a UBI device to
be available early at boot (and not only after late_initcall), so
volumes on them can be used eg. as NVMEM providers for other drivers.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>