[SERIAL] remove 8250_acpi (replaced by 8250_pnp and PNPACPI)

With the combination of PNPACPI and 8250_pnp, we no longer need 8250_acpi.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>