mmc(4), mmcsc(4): Remove missing Xref

There is no evidence of at91_mci in the code whatsoever. Most of the at91
drivers where removed in 802baf0ba66c18ca52aeaf4a3e7b05e85d8e4d3b

PR: 218970
Reported by: reezer@reezer.org

Approved by: imp@
Differential Revision: https://reviews.freebsd.org/D29193