pxa: Disable dcache on palmld, palmtc, zipitz2

These platforms don't include dcache support. Define CONFIG_SYS_DCACHE_OFF
so that functions don't try to call non-existent routines like
flush_dcache_range().

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>