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>