LPC43XX: Attempt to handle case of running WDT.

If the WDT is running, it may reset while we are trying to flash.
Since we can't disable it, we set a long period and attempt to pet it
before each IAP call.
Merge pull request #57 from aibara/dev/lpc43xx_wdt

LPC43XX: Attempt to handle case of running WDT.