[LPC1549] Fix serial interrupt issues

* Fix USART2 base address offset
* Fix issue by handling interrupt type (TxIqr and RxIrq)
* ISR now correctly refers INTSTAT instead of STAT to get interrupt
cause
* Disable interupt when update vector table
* Tested by issue #616 test cases and serial_interrupt test
Merge pull request #633 from toyowata/master

Target: LPC1549 - Fix serial interrupt issues (issue report #616)