[PARISC] Untangle <asm/processor.h> header include mess

asm/processor.h on parisc wants spinlocks for cpuinfo, but
linux/spinlock_types.h needs lockdep, and lockdep wants prefetch.

This leads to a horrible circular dependancy, because <asm/processor.h>
is including something which depends on things which are not defined
until the end of the file.

Kludge around this by moving prefetch related code into <asm/prefetch.h>
and including it before <linux/spinlock_types.h>, however this is just
a temporary solution until this mess can be cleaned up.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>