dt-bindings: mtd: add binding for BCM4908 partitions

BCM4908 uses fixed partitions layout but function of some partitions may
vary. Some devices use multiple firmware partitions and those partitions
should be marked to let system discover their purpose.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Richard Weinberger <richard@nod.at>