mtd: pxa3xx_nand: add helpers to setup the timings

Add helpers to setup the timings in the pxa3xx driver. These helpers
allow to either make use of the nand framework nand_sdr_timings or the
pxa3xx specific pxa3xx_nand_host, for compatibility reasons.

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>