doc: document sbi command

Add a man-page for the sbi command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/MAINTAINERS b/MAINTAINERS
index a7a62df..e7a6dd9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -951,6 +951,7 @@
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-riscv.git
 F:	arch/riscv/
 F:	cmd/riscv/
+F:	doc/usage/sbi.rst
 F:	drivers/timer/andes_plmt_timer.c
 F:	drivers/timer/sifive_clint_timer.c
 F:	tools/prelink-riscv.c