mmc: trace added

Defining CONFIG_MMC_TRACE in the include board file it is possible to activate
a tracing support.
This code helps in case of eMMC hw failure or to investigate possible eMMC
initialization issues.

Signed-off-by: Raffaele Recalcati <raffaele.recalcati@bticino.it>
Signed-off-by: Andy Fleming <afleming@freescale.com>