ARM: implement support for read-mostly sections

As our SMP implementation uses MESI protocols.  Grouping together data
which is mostly only read together means that we avoid unnecessary
cache line bouncing when this code shares a cache line with other data.

In other words, cache lines associated with read-mostly data are
expected to spend most of their time in shared state.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>