fan53555: attach to build and switch from syr827

Rather than extending syr827 for syr828 (as initially done in D31103)
switch to the Fairchild Semiconductor Corporation fan53555 implementation
which is in-tree but was not attached to the build. The fan53555
implementation also supports syr827/syr8278 already. [1]
Update NOTES and the arm64 GENERIC configuration for the switch.
syr827 for now stays in the tree but is not used by any
kernel configuration.

Suggested by: mmel [1]
Reviewed by: mmel, manu
Differential Revision: https://reviews.freebsd.org/D31112