mtd: bcm63xxpart: use nvram for PSI size

Read out the SPI size from nvram instead of defaulting to 64KiB — some
vendors actually use values larger than the "max" value of 64.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>