mmcsd.4: s/DESCRIPTION/HARDWARE + HISTORY + SPDX

This manual only describes a tiny amount suitable for the HW Relnotes.
Remove the description section and replace it with a hardware section
which will appear there.

MFC after: 3 minutes
Reported by: ivy
History Ref: 5bcb64f20afff21be511cc5 (Add mmc and mmcsd)