New port: sysutils/opensbi: OpenSBI, a RISC-V bootloader and firmware

OpenSBI is a RISC-V bootloader and set of firmware libraries that
implement the RISC-V Supervisor Binary Interface (SBI) required for
supervisor targets, such as the FreeBSD kernel.

This port installs the OpenSBI libraries that can be used for
development of custom firmwares, as well as the firmware files for the
QEMU virt and SiFive U540 platforms.

Reviewed by: lwhsu, philip (earlier version)
Approved by: lwhsu
Differential Revision: https://reviews.freebsd.org/D22164