mtd: bcm63xxpart: move imagetag parsing to its own parser

Move the bcm963xx Image Tag parsing into its own partition parser. This
Allows reusing the parser with different full flash parsers.

While moving it, rename it to bcm963* to better reflect it isn't chip,
but reference implementation specific.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>