target/arm: Correctly initialize MDCR_EL2.HPMN

When working with performance monitoring counters, we look at
MDCR_EL2.HPMN as part of the check whether a counter is enabled. This
check fails, because MDCR_EL2.HPMN is reset to 0, meaning that no
counters are "enabled" for < EL2.
That's in violation of the Arm specification, which states that

> On a Warm reset, this field [MDCR_EL2.HPMN] resets to the value in
> PMCR_EL0.N

That's also what a comment in the code acknowledges, but the necessary
adjustment seems to have been forgotten when support for more counters
was added.
This change fixes the issue by setting the reset value to PMCR.N, which
is four.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>