sh: Factor in cpu id for selection of cache colour fixmap.

In the SMP VIPT case the page copy/clear ops still perform colouring,
care needs to be taken that CPUs don't end up stepping on each other,
so we give them a bit of room to work with.

At the same time, we reduce the worst-case colouring given that these
pages are always consumed.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>