xen/arm: Make AFFINITY_MASK generate correct mask for level3

xen/arm: Make AFFINITY_MASK generate correct mask for level3

The original affinity shift bits algorithm in AFFINITY_MASK is buggy,
it could not generate correct affinity shift bits of level3.
The macro MPIDR_LEVEL_SHIFT can calculate level3 affinity shift bits
correctly. We use this macro in AFFINITY_MASK to generate correct
mask for level3.

Signed-off-by: Wei Chen <Wei.Chen@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>