src: Never overwrite fadt->flags

Instead, just flip the desired bits using bitwise operations. As this is
initially zero, the resulting value is the same. This allows flags to be
set from anywhere regardless of execution order.

Tested on Asus P8Z77-V LX2 with Linux 5.7.6 and Windows 10 at the end of
the patch train, both operating systems are able to boot successfully.

Change-Id: Icfd580a20524936cd0adac574331b09fb2aea925
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43387
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/quark/acpi.c

@@ -25,7 +25,7 @@ void acpi_fill_fadt(acpi_fadt_t *fadt)
 	uint32_t pmbase = pci_read_config32(dev, R_QNC_LPC_PM1BLK)
 		& B_QNC_LPC_PM1BLK_MASK;
 
-	fadt->flags = ACPI_FADT_RESET_REGISTER | ACPI_FADT_PLATFORM_CLOCK;
+	fadt->flags |= ACPI_FADT_RESET_REGISTER | ACPI_FADT_PLATFORM_CLOCK;
 
 	/* PM1 Status: ACPI 4.8.3.1.1 */
 	fadt->pm1a_evt_blk = pmbase + R_QNC_PM1BLK_PM1S;