Clock: Static: rename sink frequency member to Freqeuncy

Naming it the same as the class caused a gcc error:
'SystemClock' has the same name as the
class in which it is declared
Uart: STM32: replace STM32_APBx_FREQUENCY by clockSource::Uartx
I2c: STM32: replaced STM32_APB1_FREQUENCY with clockSource::I2cx
Timer: STM32: setPeriod now uses clockSource for calculations