mtd: nand: pxa3xx: Split prepare_command_pool() in two stages

This commit splits the prepare_command_pool() function into two
stages: prepare_start_command() / prepare_set_command().

This is a preparation patch without any functionality changes,
and is meant to allow support for multiple page reading/writing
operations.

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>