intel/skylake: disable ACPI PM Timer to enable XTAL OSC shutdown

Keeping ACPI PM timer alive prevents XTAL OSC shutdown in S0ix
which has a power impact.

Based on a DT variable, this patch disables the ACPI PM timer
late in the boot sequence - disabling earlier will lead to a hang
since the FSP boot flow needs this timer. This also hides the ACPI PM
timer from the OS by removing from FADT table. Once the ACPI PM timer
is disabled, TCO gets switched off as well.

BRANCH=none
BUG=chrome-os-partner:48646
TEST=Build for skylake board with the PmTimerDisabled policy in
devicetree set to 1.
iotools mmio_read32 0xfe0000fc should return 0x2.
cat /sys/devices/system/clocksource/clocksource0/available_clocksource
should list only "tsc hpet". acpi_pm should be removed from this list.

Change-Id: Icfdc51bc33b5190a55196d67e18afdaaa2f9b310
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 18bcb8a434b029295e1f1cc925e2b47e79254583
Original-Change-Id: Ifebe8bb5a7978339e07e4e12e174b9b978135467
Original-Signed-off-by: Archana Patni <archana.patni@intel.com>
Original-Signed-off-by: Subramony Sesha <subramony.sesha@intel.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/319361
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/13588
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

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