mtd: pxa3xx_nand: add a default chunk size

When keeping the configuration set by the bootloader (by using
the marvell,nand-keep-config property), the pxa3xx_nand_detect_config()
function is called and set the chunk size to 512 as a default value if
NDCR_PAGE_SZ is not set.

In the other case, when not keeping the bootloader configuration, no
chunk size is set. Fix this by adding a default chunk size of 512.

Fixes: 70ed8523 ("mtd: nand: pxa3xx: Introduce multiple page I/O
support")

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>