[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>