[HAL][K20D50M] Serial baudrate improvements / clk_freqs.h fix

Three changes, first it fixes:
https://github.com/mbedmicro/mbed/issues/761 (which was reported
slightly wrong because K20 has again different clocking from KLXX for
uarts).

Second it adds mcgpllfll to clk_freqs, which again is different for K20
compared to KLXX .

Finally it adds the fractional baudrate divider for more accurate serial
baudrates.

Downside of all this: High frequency communication via USBTX/USBRX doesn't work properly since the interface chip isn't use the fractional divider so has inaccurate baudrate. However users can always give an offset to the baudrate to 'fix' this.

Also this one can be automatically merged :D. Pretty sure stuff is correct now, but at least better than it was.