mtd: nand: pxa3xx: Add helper function to set page address

Let's simplify the code by first introducing a helper function
to set the page address, as done by the READ0, READOOB and SEQIN
commands.

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>