sysutils/opensbi: update to v1.8.1

In this update, drop the PMP workaround patch for FU540. This will be
fixed in the src tree.

As usual, bump dependent u-boot ports.

Tested by: Klaus Küchemann <maciphone2@googlemail.com>
Reviewed by: lwhsu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54421