x86, early-printk: Update earlyprintk documentation (and kill x86 copy)

Documentation/kernel-parameters.txt and
Documentation/x86/x86_64/boot-options.txt contain virtually
identical text describing earlyprintk.

This consolidates the two copies and updates the documentation a
bit.  No one ever documented the:

	earlyprintk=serial,0x1008,115200

syntax, nor mentioned that ARM is now a supported earlyprintk
arch.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rob Landley <rob@landley.net>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20130410210338.E2930E98@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/kernel-parameters.txt

@@ -757,19 +757,31 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			(mmio) or 32-bit (mmio32).
 			The options are the same as for ttyS, above.
 
-	earlyprintk=	[X86,SH,BLACKFIN]
+	earlyprintk=	[X86,SH,BLACKFIN,ARM]
 			earlyprintk=vga
 			earlyprintk=xen
 			earlyprintk=serial[,ttySn[,baudrate]]
+			earlyprintk=serial[,0x...[,baudrate]]
 			earlyprintk=ttySn[,baudrate]
 			earlyprintk=dbgp[debugController#]
 
+			earlyprintk is useful when the kernel crashes before
+			the normal console is initialized. It is not enabled by
+			default because it has some cosmetic problems.
+
 			Append ",keep" to not disable it when the real console
 			takes over.
 
 			Only vga or serial or usb debug port at a time.
 
-			Currently only ttyS0 and ttyS1 are supported.
+			Currently only ttyS0 and ttyS1 may be specified by
+			name.  Other I/O ports may be explicitly specified
+			on some architectures (x86 and arm at least) by
+			replacing ttySn with an I/O port address, like this:
+				earlyprintk=serial,0x1008,115200
+			You can find the port for a given device in
+			/proc/tty/driver/serial:
+				2: uart:ST16650V2 port:00001008 irq:18 ...
 
 			Interaction with the standard serial driver is not
 			very good.

Documentation/x86/x86_64/boot-options.txt

@@ -91,20 +91,6 @@ APICs
 		 apicmaintimer. Useful when your PIT timer is totally
 		 broken.
 
-Early Console
-
-   syntax: earlyprintk=vga
-           earlyprintk=serial[,ttySn[,baudrate]]
-
-   The early console is useful when the kernel crashes before the
-   normal console is initialized. It is not enabled by
-   default because it has some cosmetic problems.
-   Append ,keep to not disable it when the real console takes over.
-   Only vga or serial at a time, not both.
-   Currently only ttyS0 and ttyS1 are supported.
-   Interaction with the standard serial driver is not very good.
-   The VGA output is eventually overwritten by the real console.
-
 Timing
 
   notsc