mtd: bcm63xxpart: Remove dependency on mach-bcm63xx

Read nvram directly from flash instead of using the in-memory copy that
mach-bcm63xx has, to remove the dependency on mach-bcm63xx and allow the
parser to work on bmips too.

Rename remaining BCM63XX defines to BCM963XX as these are properties of
the flash layout on the board.

BCM963XX_DEFAULT_PSI_SIZE changes from SZ_64K to 64 because it will be
multiplied by SZ_1K later on.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>