x86: acpi: Add function for querying GPE status

Add a function that can be implemented by the SOC to read
and clear the status of a single GPE.  This can be used
during firmware to poll for interrupt status.

BUG=chrome-os-partner:53336

Change-Id: I551276f36ff0d2eb5b5ea13f019cdf4a3c749a09
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/16669
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/acpi.c

@@ -1093,3 +1093,8 @@ void acpi_save_gnvs(u32 gnvs_address)
 	if (gnvs)
 		*gnvs = gnvs_address;
 }
+
+__attribute__ ((weak)) int acpi_get_gpe(int gpe)
+{
+	return -1; /* implemented by SOC */
+}