mtd: pxa2xx-flash: switch back from memremap to ioremap_cached

This reverts commit 06968a54 ("mtd: pxa2xx-flash: switch from
ioremap_cache to memremap"), since NOR with memory semantics in array mode
and RAM are not necessarily the same thing, and architectures may implement
ioremap_cached() and memremap() with different memory attributes.

For this reason, ioremap_cached() has been brought back from the dead on
the ARM side, so switch this driver back to using it instead of memremap().

Cc: David Woodhouse <dwmw2@infradead.org>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>