Documentation: admin-guide: add earlycon documentation for RISC-V

Kernels booting on RISC-V can specify "earlycon" with no options on
the Linux command line, and the generic DT earlycon support will query
the "chosen/stdout-path" property (if present) to determine which
early console device to use.  Document this appropriately in the
admin-guide.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andreas Schwab <schwab@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -988,6 +988,10 @@
 			chosen node or the ACPI SPCR table if supported by
 			the platform.
 
+			[RISCV] When used with no options, the early
+			console is determined by the stdout-path
+			property in the device tree's chosen node.
+
 		cdns,<addr>[,options]
 			Start an early, polled-mode console on a Cadence
 			(xuartps) serial port at the specified address. Only