Documentation: document earlycon without options for more platforms

The earlycon options without arguments is supposed to work on all
device tree platforms, not just arm64.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -983,12 +983,10 @@
 
 	earlycon=	[KNL] Output early console device and options.
 
-			[ARM64] The early console is determined by the
-			stdout-path property in device tree's chosen node,
-			or determined by the ACPI SPCR table.
-
-			[X86] When used with no options the early console is
-			determined by the ACPI SPCR table.
+			When used with no options, the early console is
+			determined by stdout-path property in device tree's
+			chosen node or the ACPI SPCR table if supported by
+			the platform.
 
 		cdns,<addr>[,options]
 			Start an early, polled-mode console on a Cadence