upd7810: use intermediate flip flops for extended timer/event counter

The extended timer/event counter uses intermediate flip flops (LV0 and
LV1) before writing the output to CO0 and CO1.

The introduction of LV0/LV1 addresses three issues:
- Level inversion occurs after the flip flop is output, not before;
- The intermediate flip flop can be altered without affecting the
  output of CO0/CO1;
- upd7810_write_EOM() didn't work properly when the level inversion
  bit was set along other bits.