hw/misc/mps2-fpgaio: Implement PSCNTR and COUNTER

hw/misc/mps2-fpgaio: Implement PSCNTR and COUNTER

In the MPS2 FPGAIO, PSCNTR is a free-running downcounter with
a reload value configured via the PRESCALE register, and
COUNTER counts up by 1 every time PSCNTR reaches zero.
Implement these counters.

We can just increment the counters migration subsection's
version ID because we only added it in the previous commit,
so no released QEMU versions will be using it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180820141116.9118-3-peter.maydell@linaro.org