sdhci: fdt: Remove sdhci_generic compatible string

This was used when we had our own DTS, it's not used anymore.

Reviewed-by: bz
Differential Revision: https://reviews.freebsd.org/D41807
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 8c7e747491ad636d6ee4069a74ddb24814870540)