opensbi: bump to 1.8.1

opensbi: bump to 1.8.1

This intermediate release has following fixes:
- Typo fix in comments of sbi_hartindex_to_hartid()
- Fix hart protection abstraction for platforms without PMP

Allow the platform to boot without PMP, but it is still necessary to
enable PMP on QEMU; otherwise, there will be nothing to protect the
M-mode firmware (OpenSBI) from lower privilege modes.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Tested-by: Michael Opdenacker <michael.opdenacker@rootcommit.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>