hw/misc/mps2-fpgaio: Fold counters subsection into main vmstate

hw/misc/mps2-fpgaio: Fold counters subsection into main vmstate

We've already broken migration compatibility for all the MPS
boards, so we might as well take advantage of this to simplify
the vmstate for the FPGAIO device by folding the counters
subsection into the main vmstate description.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210219144617.4782-38-peter.maydell@linaro.org