[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.
Merge pull request #774 from Sissors/ksdk

Targets: K20D50M - Serial baudrate improvements / clk_freqs.h fix