lpc176x: Add an enable_peripheral_clock() helper function

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

src/lpc176x/serial.c

@@ -16,7 +16,7 @@ serial_init(void)
 {
     // Setup baud
     LPC_UART0->LCR = (1<<7); // set DLAB bit
-    LPC_SC->PCLKSEL0 = (LPC_SC->PCLKSEL0 & ~(0x3<<6)) | (0x1<<6);
+    enable_peripheral_clock(PCLK_UART0);
     uint32_t pclk = SystemCoreClock;
     uint32_t div = pclk / (CONFIG_SERIAL_BAUD * 16);
     LPC_UART0->DLL = div & 0xff;