dm: sandbox: Switch over to use DM for block devices

Now that the drivers used by sandbox support CONFIG_BLK, we can switch
sandbox over to use driver model for block devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>

configs/sandbox_defconfig

@@ -28,6 +28,7 @@ CONFIG_SPL_SYSCON=y
 CONFIG_DEVRES=y
 CONFIG_ADC=y
 CONFIG_ADC_SANDBOX=y
+CONFIG_BLK=y
 CONFIG_CLK=y
 CONFIG_SANDBOX_GPIO=y
 CONFIG_SYS_I2C_SANDBOX=y