ARM: mxs: allow boards to select DC-DC switching clock source

For some board designs, it might be useful to switch the DC-DC
clock source to something else rather the default 24 MHz, e.g.
for EMI reasons.

For this, override the mxs_power_setup_dcdc_clocksource function
in your board support files.

Example:
void mxs_power_setup_dcdc_clocksource(void)
{
    mxs_power_switch_dcdc_clocksource(POWER_MISC_FREQSEL_20MHZ);
}

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>