mtd: pxa3xx_nand: rework flash detection and timing setup

Rework the pxa3xx_nand driver to allow using functions exported by the
nand framework to detect the flash and the timings. Then setup the
timings using the helpers previously added.

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>