mmc: sdhci-of-arasan: Modified SD default speed to 19MHz for ZynqMP

SD standard speed timing was met only at 19MHz and not 25 MHz, that's
why changing driver to 19MHz. The reason for this is when a level shifter
is used on the board, timing was met for standard speed only at 19MHz.
Since this level shifter is commonly required for high speed modes,
the driver is modified to use standard speed of 19Mhz.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/1623753837-21035-2-git-send-email-manish.narani@xilinx.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>