ACPI: Work around IASL warning reading/writing same register

ACPI: Work around IASL warning reading/writing same register

The newer versions of IASL are unhappy when an operator
has the same object as both source and destination.

The warning can be completely disabled with a command
line argument, but in general, I'd really rather not
just disable warnings.

The bits in this register are write 1 to clear, so reading and
writing the same register is what we want to do.  Instead, store
it in a temporary register then write it in a second operation.

Fixes warning:
dsdt.aml   1396:  Store(PWST, PWST)
Warning  3023 -                 ^ Duplicate value in list
(Source is the same as Target)

Change-Id: I52d73d4431db237be83016d67cd397f31b53d9c6
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12691
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>