sysutils/opensbi: update to version 0.6

This update adds a new default option: SPIKE. This builds support for the Spike
platform. Also, the toolchain dependency has been switched from riscv64-gcc to
riscv64-none-elf-gcc.

This release has:

Relocation lottery in reference firmwares
Preferred boot HART field in fw_dynamic_info
Platform specific optional way to specify MISA
Hypervisor v0.5 specification support
SBI v0.2 TIME extension
SBI v0.2 IPI extension
SBI v0.2 RFENCE extension
T-HEAD C910 platform support
HART exit path support
Dynamic creation of IPI events
Dynamic registration of SBI extensions
Removed qemu/sifive_u platform support
Spike platform support
Print MIDELEG and MEDELEG in boot prints

Reviewed by: lwhsu
Approved by: lwhsu
Differential Revision: https://reviews.freebsd.org/D23820