mtd: nand: pxa3xx: Consolidate ECC initialization

In order to avoid code duplication, let's consolidate the ECC setting
for all SoC variants. Such decision is based on page size and ECC
strength requirements.

Also, provide a default value for the case where such ECC information
is not provided (non-ONFI devices).

Tested-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>