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/simulator/Kconfig

@@ -15,4 +15,12 @@ config SIMULATOR_SELECT
     select HAVE_GPIO_SPI
     select HAVE_GPIO_HARD_PWM
 
+config CLOCK_FREQ
+    int
+    default 20000000
+
+config SERIAL_BAUD
+    int
+    default 250000
+
 endif