mtd: pxa3xx_nand: fix some compile issues on non-ARM arches

Using readsl() result in a build error on i386. Fix this by using
ioread32_rep() instead, to allow compile testing the pxa3xx nand driver
on other architectures later.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
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>