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: NRob Herring <robh@kernel.org>
Signed-off-by: NJonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>