Add ATA PIO IRQ handler, documentation

Still WIP, doesn't look like IRQ handler works yet.
diff --git a/include/kernel/registers.h b/include/kernel/registers.h
index 687bd0e..8abe32f 100644
--- a/include/kernel/registers.h
+++ b/include/kernel/registers.h
@@ -2,6 +2,9 @@
 
 #include "kint.h"
 
+/**
+ * Represents the full execution state before an interrupt.
+ */
 struct registers
 {
 	uint ds;