MIPS: bcm963xx: Move extended flash address to bcm_tag header file

MIPS: bcm963xx: Move extended flash address to bcm_tag header file

The extended flash address needs to be subtracted from bcm_tag flash
image offsets. Move this value to the bcm_tag header file.

Renamed define name to consistently use bcm963xx for flash layout
which should be considered a property of the board and not the SoC
(i.e. bcm63xx could theoretically be used on a board without CFE
or any flash).

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc: MIPS Mailing List <linux-mips@linux-mips.org>
Cc: MTD Maling List <linux-mtd@lists.infradead.org>
Patchwork: https://patchwork.linux-mips.org/patch/11833/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>