powerpc/selftest: Add gettimeofday() benchmark

This adds a benchmark directory to the powerpc selftests and adds a
gettimeofday() benchmark to it.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>