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>
diff --git a/arch/arm/include/asm/arch-mxs/sys_proto.h b/arch/arm/include/asm/arch-mxs/sys_proto.h
index 20ff101..f2b075e 100644
--- a/arch/arm/include/asm/arch-mxs/sys_proto.h
+++ b/arch/arm/include/asm/arch-mxs/sys_proto.h
@@ -25,6 +25,8 @@
 void mxs_common_spl_init(const uint32_t arg, const uint32_t *resptr,
 			 const iomux_cfg_t *iomux_setup,
 			 const unsigned int iomux_size);
+
+void mxs_power_switch_dcdc_clocksource(uint32_t freqsel);
 #endif
 
 struct mxs_pair {