amd/stoneyridge: Prepare for vboot rebooting system

Implement the function vboot_platform_prepare_reboot() which is normally
a weak function.

The SlpTyp field of the PM1 register is not reset to its default value
when the APU restarts.  This change prevents a failing condition if
vboot decides to reset the system instead of allowing an S3 resume to
continue.

TEST=Resume Grunt when vboot attempts a reset, verify a fresh boot instead
BUG=b:117089826

Change-Id: I6e0e3e541bad89ca5b23d6ddb6e5c0df7f762f10
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/28877
Reviewed-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/stoneyridge/pmutil.c

@@ -32,3 +32,15 @@ int vboot_platform_is_resuming(void)
 
 	return acpi_sleep_from_pm1(inw(pm_acpi_pm_cnt_blk())) == ACPI_S3;
 }
+
+/* If vboot requests a system reset, modify the PM1 register so it will never be
+ * misinterpreted as an S3 resume. */
+void vboot_platform_prepare_reboot(void)
+{
+	uint16_t pm1;
+
+	pm1 = inw(pm_acpi_pm_cnt_blk());
+	pm1 &= ~SLP_TYP;
+	pm1 |= SLP_TYP_S5 << SLP_TYP_SHIFT;
+	outw(pm1, pm_acpi_pm_cnt_blk());
+}