mtd: nand: pxa3xx: Support command buffer #3

Some newer controllers support a fourth command buffer. This additional
command buffer allows to set an arbitrary length count, using the
NDCB3.NDLENCNT field, to perform non-standard length operations
such as the ONFI parameter page read.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>