drivers: Add SPDX identifiers to files missing them

This adds SPDX identifiers to the remaining source files in the
drivers directory that don't already have them.

Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Change-Id: I97f96de857515214069c3b77f3c781f7f0555c6e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66499
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/drivers/i2c/ptn3460/chip.h b/src/drivers/i2c/ptn3460/chip.h
index 8bd6d9e..2bf7e5e 100644
--- a/src/drivers/i2c/ptn3460/chip.h
+++ b/src/drivers/i2c/ptn3460/chip.h
@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 struct drivers_i2c_ptn3460_config {
 
 };