mci: sdhci: atmel: extend driver for PBL usage ...

The BootROM resets both the SD/MMC host controller and the pin controller,
but the card itself remains in transfer mode. If we redo host-side
setup, we can directly read new blocks off the card.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>