configs: sheevaplug: enable driver-model based MMC

The non-dm MMC driver has been converted to the driver model. The
sheevaplug was using the non-dm driver and this commit enables the new
driver.

Signed-off-by: Harm Berntsen <harm.berntsen@nedap.com>
Reviewed-by: Stefan Roese <sr@denx.de>
CC: Prafulla Wadaskar <prafulla@marvell.com>
CC: Stefan Roese <sr@denx.de>
CC: Tom Rini <trini@konsulko.com>