[PATCH] ia64: note requirement for 8250_pnp, now that 8250_acpi is gone

We removed 8250_acpi in 2.6.17.  If we don't have PNPACPI turned on, we
won't find any ACPI serial devices, so mention this requirement in the
troubleshooting part of the documentation.

CONFIG_PNPACPI is already turned on in all the relevant defconfigs.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/ia64/serial.txt

@@ -124,6 +124,13 @@ TROUBLESHOOTING SERIAL CONSOLE PROBLEMS
 
 	- Add entry to /etc/securetty for console tty.
 
+    No ACPI serial devices found in 2.6.17 or later:
+
+	- Turn on CONFIG_PNP and CONFIG_PNPACPI.  Prior to 2.6.17, ACPI
+	  serial devices were discovered by 8250_acpi.  In 2.6.17,
+	  8250_acpi was replaced by the combination of 8250_pnp and
+	  CONFIG_PNPACPI.
+
 
 
 [1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf