aspeed: Change type of eMMC device

aspeed: Change type of eMMC device

The QEMU device model representing the eMMC device of the machine is
currently created with type SD_CARD. Change the type to EMMC now that
it is available.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Tested-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>