mtd: pxa3xx_nand: clean up the pxa3xx timings

With the previous modifications, lots of pxa3xx specific definitions can
be removed.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>