davinci, mmc: Added a delay reading ext CSD register

Without this additional delay, some eMMC don't negotiate properly bus width
Tested on:
 - Toshiba THGBM2G8D8FBAIB
 - Toshiba THGBM4G4D1HBAR
 - Micron MTFC4GMVEA (the one giving the problem)
 - Hynix H26M64002BNR
 - SanDisk SDIN5E1-32G

Signed-off-by: Davide Bonfanti <davide.bonfanti@bticino.it>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>