Documentation: Advertise support for OpenSBI

Change-Id: Ie990bb95fcdcfab0246e8c694704022d9b8b5195
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34690
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Philipp Hug <philipp@hug.cx>
Reviewed-by: Xiang Wang <merle@hardenedlinux.org>

Documentation/mainboard/sifive/hifive-unleashed.md

@@ -17,7 +17,6 @@ The following things are still missing from this coreboot port:
 - Provide serial number to payload (e.g. in device tree)
 - Implement instruction emulation
 - Support for booting Linux on RISC-V
-- Add support to run OpenSBI payload in m-mode
 - SMP support in trap handler
 
 ## Configuration