As per MMC spec, once power has been applied to an SD card, the card
can take as much as 250ms to complete its power-up cycle, and become
responsive to CMD0.
When this delay was not in place, activating the SD card in the env
init failed sometimes. With it, no more failure are observed.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>