cpu: Add new asidx_from_attrs() method

cpu: Add new asidx_from_attrs() method

Add a new method to CPUClass which the memory system core can
use to obtain the correct address space index to use for a memory
access with a given set of transaction attributes, together
with the wrapper function cpu_asidx_from_attrs() which implements
the default behaviour ("always use asidx 0") for CPU classes
which don't provide the method.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>