hw/misc/mps2-fpgaio: Implement 1Hz and 100Hz counters

The MPS2 FPGAIO block includes some simple free-running counters.
Implement these.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180820141116.9118-2-peter.maydell@linaro.org

include/hw/misc/mps2-fpgaio.h

@@ -38,6 +38,10 @@ typedef struct {
     uint32_t misc;
 
     uint32_t prescale_clk;
+
+    /* These hold the CLOCK_VIRTUAL ns tick when the CLK1HZ/CLK100HZ was zero */
+    int64_t clk1hz_tick_offset;
+    int64_t clk100hz_tick_offset;
 } MPS2FPGAIO;
 
 #endif