pxa: turn icache off in cpu_init_crit()

The comment in the low-level initialization function cpu_init_crit() says that
the caches are being disabled, but (oddly) the icache is actually turned on.
This is probably not a good idea prior to relocating code, so this patch turns
it off.  Tested on the pxa270.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>