arch/x86/acpigen: Provide helper functions for enabling/disabling GPIO

In order to allow GPIOs to be set/clear according to their polarity,
provide helper functions that check for polarity and call set/clear
SoC functions for generating ACPI code.

BUG=None
BRANCH=None
TEST=Verified that the ACPI code generated remains the same as before
for reef.

Change-Id: Ie8bdb9dc18e61a4a658f1447d6f1db0b166d9c12
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/18427
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

Documentation/acpi/gpio.md

@@ -3,6 +3,7 @@
 # Table of contents #
 - Introduction
 - Platform Interface
+- Helper routines
 - Implementation details
 - Arguments and Local Variables Management
 
@@ -55,6 +56,23 @@ adding them as AML code callbacks for the following reasons:
 3. Allows GPIO AML methods to be present under any device scope and
    gives SoC the flexibility to call them without any restrictions.
 
+# Helper routines #
+
+In order to relieve drivers of the task of implementing the same code
+for enabling/disabling Tx GPIOs based on the GPIO polarity, helper
+routines are provided which implement this common code and can be used
+directly in the driver routines:
+1. Enable Tx GPIO
+   int acpigen_enable_tx_gpio(struct acpi_gpio gpio)
+2. Disable Tx GPIO
+   int acpigen_disable_tx_gpio(struct acpi_gpio gpio)
+
+Both the above functions take as input struct acpi_gpio type and
+return -1 on error and 0 on success. These helper routines end up
+calling the platform specific acpigen_soc_{set,clear}_tx_gpio
+functions internally. Thus, all the ACPI AML calling conventions for
+the platform functions apply to these helper functions as well.
+
 # Implementation Details #
 
 ACPI library in coreboot will provide weak definitions for all the
@@ -84,7 +102,6 @@ variables.
  acpigen_soc_clear_tx_gpio    Generate ACPI AML code to      Error = -1
                               set Tx to 0.                   Success = 0
 
-
 Ideally, the operation column in the above table should use one or
 more functions implemented by the platform in AML code library (like
 gpiolib.asl). In the example below SPC0 and GPC0 need to be