kselftest/arm64: Raise default number of loops in fp-pidbench

When fp-pidbench was originally written SVE hardware was not widely
available so it was useful to run it in emulation and the default number
of loops was set very low, running for less than a second on actual
hardware. Now that SVE hardware is reasonably available it is very much
less interesting to use emulation, bump the default number of loops up to
even out a bit of the noise on real systems. On the machine I have to hand
this now takes about 15s which is still a toy microbenchmark but perhaps a
bit more useful.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>