Re-introduce a config switch (l2x0_write_allocate=) for controlling
the write-allocate policy. Such policy is known to induce very high
latencies: it typically doubles the worst-case figures on imx6q (PL310
cache controller).

By default, the default policy is usually determined by the AWCACHE
attribute settings unless the platform code forced it in the auxiliary
control register. In the common case, write allocation is selected.

The patch applies to L310 and L220 cache controllers exclusively:

- if l2x0_write_allocate= is not set, turn write allocation off.

- if write allocation ends up being enabled, warn about expected high
  latencies.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Signed-off-by: Philippe Gerum <rpm@xenomai.org>