This patch allows boards to select a max ADC clock speed. This could be
handy if the board wants to clock the ADC differently according to the
board's front end analog circuitry or MCU model's ADC capabilities.
The current implementation uses the core clock frequency to calculate
the needed prescalar to achieve a given ADC clock frequency. This is
incorrect. This patch fixes the calculation to use the correct source
clock (PCKLK2 ie APB2). It also changes the defined max clock rate to
use the frequency macro to improve readability.
