dfu: Sort Kconfig entries alphabetically

The DFU_MTD Kconfig entry is in the wrong position, move it into the
correct alphabetically sorted position. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>