ARM: pxa: remove support for MTD_XIP

Using MTD-XIP does not work on multiplatform kernels because
it requires SoC specific register accesses to be done from
low-level flash handling functions in RAM while the rest of the
kernel sits in flash.

I found no evidence of anyone still actually using this feature,
so remove it from PXA to avoid spending a lot of time on
actually making it work.

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