Create a sun7i-a20-bananapi.dtb hard link to bananapi.dtb to fix
a boot failure on the Banana Pi SoC.

This is a direct commit to stable/11, as the sun7i-a20-bananapi.dtb
file exists in head, but appears to have been part of a larger
rework of dtb-related files that may have larger consequences than
hard link creation. Note: creating a hard link to dtb files was
an original fix in 12-CURRENT beforehand, introduced in r319603.

Approved by: re (marius)
Sponsored by: The FreeBSD Foundation