aarch64: Adjust big.LITTLE tune files to use -mcpu

mcpu is more appropriate on aarch64 and generates more optimized code
forr a given SOC, unlike -march/-mtune combination as decribed here [1]

[1] https://community.arm.com/developer/tools-software/tools/b/tools-software-ides-blog/posts/compiler-flags-across-architectures-march-mtune-and-mcpu



Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>