Added MCGPLLFLL clock function, squished some 'bugs'

PWM bug mentioned here:
https://mbed.org/questions/2315/BUG-Exporting-to-ARM-GCC-does-not-work-n/

Fast internal osc prescaler wasn't taken into account at us_ticker
interrupt clock source, now it is.

And the MCGPLLFLL output for peripherals is now properly calculated for UART0 and PWM.