Documentation: Add "earlycon=sbi" to the admin guide

This argument is supported on RISC-V systems and widely used, but was
not documented here.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -1044,6 +1044,10 @@
 			specified address. The serial port must already be
 			setup and configured. Options are not yet supported.
 
+		sbi
+			Use RISC-V SBI (Supervisor Binary Interface) for early
+			console.
+
 		smh	Use ARM semihosting calls for early console.
 
 		s3c2410,<addr>