PIT: add alternative faster PIT

This is enabled by default on 486+ CPUs and can be forced disabled/enabled with pit_mode=0/1

src/device/keyboard_xt.c

@@ -535,7 +535,7 @@ kbd_write(uint16_t port, uint8_t val, void *priv)
 
 		if (speaker_enable)
 			was_speaker_enable = 1;
-		pit_ctr_set_gate(&pit->counters[2], val & 1);
+		pit_devs[0].set_gate(pit_devs[0].data, 2, val & 1);
 
 		if (val & 0x80) {
 			kbd->pa = 0;