Added check in DTS for a flag indicating that the SD signals for Zynq's SDHCI host
have been routed through the EMIO interface instead of the MIO.
When connected directly to MIO pins, the SDHCI can run at 50MHz. When
routed through the EMIO interconnect, the maximum SD clock is 25MHz.

Signed-off-by: Joshua Lamorie <joshua.lamorie@xiphos.com>