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 <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Andreas Schwab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			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