simulator: Rework dummy simulator code to user timer_irq / serial_irq

Change the simulator to use the generic timer_irq.c and serial_irq.c
code for (dummy) timer and io handling.  This is just to make the code
a better example for other developers (most micro-controllers will use
the timer_irq.c and serial_irq.c code).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/serial_irq.h

@@ -1,6 +1,8 @@
 #ifndef __GENERIC_SERIAL_IRQ_H
 #define __GENERIC_SERIAL_IRQ_H
 
+#include <stdint.h> // uint32_t
+
 // callback provided by board specific code
 void serial_enable_tx_irq(void);