Commit 07a8e3253a2d8a2076c9c83c4ed4158fa3fbb2a2 removes
CONFIG_MMC_VOLUME_NAME from the Kconfig space. Instead we need to use
the device tree to find the "disk-name" property of "zephyr,mmc-disk"
devices.
Signed-off-by: Detlev Zundel <dzu@member.fsf.org>