K20 general and timer adjustments

Some bugs squished and interrupt code added to be compatible with current mbed api.

The K20 cannot chain its PIT timers, so no option to get 1us tick rate for them. Solution for the 'timer' timer was to create an interrupt every 1us and use a software timer. Now it uses almost the complete 32-bit range of the timer, and only does the last few bits in software. Downside: Software and hardware timer need to stay synced. Upside, don't need an interrupt every 1us, timer not working in irq context, etc.

Also the 'ticker' timer is moved to a PIT channel. 32-bit with no prescaler is better than 16-bit with prescaler. Especially since the K20 is a core-M4, and has no problems with integer multiplications and divisions (contrary to M0s).