mtd: nand: pxa3xx: Early variant detection

In order to customize early settings depending on the detected SoC variant,
move the detection to be before the nand_chip struct filling.

In a follow-up patch, this change is needed to detect the variant *before*
the call to alloc_nand_resource(), which allows to set a different cmdfunc()
for each variant.

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>