Intel BD82x6x: LPC: Unify I/O APIC setup

Intel BD82x6x: LPC: Unify I/O APIC setup

Remove local copies of reading and writing I/O APIC registers by
using already available functions.

This change is similar to

    commit db4f875a412e6c41f48a86a79b72465f6cd81635
    Author: Kyösti Mälkki <kyosti.malkki@gmail.com>
    Date:   Tue Jan 31 17:24:12 2012 +0200

        IOAPIC: Divide setup_ioapic() in two parts.

        Reviewed-on: http://review.coreboot.org/300

and

    commit e614353194c712a40aa8444a530b2062876eabe3
    Author: Kyösti Mälkki <kyosti.malkki@gmail.com>
    Date:   Tue Feb 26 17:24:41 2013 +0200

        Unify setting 82801a/b/c/d IOAPIC ID

        Reviewed-on: http://review.coreboot.org/2532

and uses `io_apic_read()` and `io_apic_write()` too. Define
`ACPI_EN` in the header file `pch.h`.

As commented by Aaron Durbin, a separate `pch_enable_acpi()` is
not needed: “The existing code path *in this file* is about enabling
the io apic.” [1].

[1] http://review.coreboot.org/#/c/3182/4/src/southbridge/intel/lynxpoint/lpc.c

Change-Id: I4478b1902d09061ca1db8eab6b71fef388c7a74c
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3183
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>