mtd: brcmnand: Add support for v6.2 controllers

Document and match the brcm,brcmnand-v6.2 compatible string, the controller has
a register layout identical to the v6.0 version and supports prefetch. Update the
command shift logic to account for v6.2 controller which are the first ones to
use a shift of 0 (6.1 used a shift of 24).

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>