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This commit improves the SDHost driver for RPi3 as following:
 * Unblock MMC_CMD(17). Using MMC_CMD(17) is more efficient on
   block reading.
 * In some low probability that SEND_OP_COND might results CRC7
   error. We can consider that the command runs correctly. We don't
   need to retry this command so removing the code for retry.
 * Using MMC_BUS_WIDTH_1 as MMC default value to improve the stability.
 * Increase the clock to 50Mhz in data mode to speed up the io.
 * Change the pull resistors configuration to gain more stability.
Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>

This commit adds SDHost driver for RaspberryPi3. SDHost driver uses the
GPIO driver to connect the SDCard and SDHost. By using this driver it is
able to read/write raw blocks on SDCard.
Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>