lpc176x: Divide system clock within timer; not on system bus

Scaling the timer on the system bus has a notable impact on
performance.  So, scale the timer within the timer hardware itself.

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

src/lpc176x/timer.c

@@ -37,6 +37,9 @@ timer_init(void)
 {
     // Disable timer
     LPC_TIM0->TCR = 0x02;
+    // Setup clock and prescaler (divide sys clock by 4)
+    LPC_SC->PCLKSEL0 = (LPC_SC->PCLKSEL0 & ~(0x3<<2)) | (0x1<<2);
+    LPC_TIM0->PR = 3;
     // Enable interrupts
     NVIC_SetPriority(TIMER0_IRQn, 2);
     NVIC_EnableIRQ(TIMER0_IRQn);