Specify .cpu cortex-a15

When building with clang without this change, as errors out saying pldw
is an unsupported instruction (because it isn't part of the ARMv7 core
instruction set).
Let as know using pldw is fine.

Change-Id: Ie1f9c4b873e93ab2b3b374d2d46e476a4e581710
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
Merge "Specify .cpu cortex-a15"