sysutils/opensbi: update to version 1.2

With this version, stop providing the library and header files
altogether. I do not think they are useful at this time.

Bump associate u-boot port revisions.

Reviewed by: lwhsu
Approved by: lwhsu (ports)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37934