Missing SPDX license references in drivers source files.

[kestewart]

Describe the bug
Missing SPDX license reference in the source file:

To Reproduce
See headers in the following files:

zephyr-master/drivers/display/Kconfig.grove
zephyr-master/drivers/entropy/Kconfig.native_posix
zephyr-master/drivers/flash/Kconfig.mcux
zephyr-master/drivers/flash/Kconfig.nios2_qspi
zephyr-master/drivers/flash/Kconfig.nrf
zephyr-master/drivers/gpio/Kconfig.cc32xx
zephyr-master/drivers/ieee802154/Kconfig.cc1200
zephyr-master/drivers/interrupt_controller/Kconfig.multilevel.aggregator_template
zephyr-master/drivers/ieee802154/Kconfig.cc1200
zephyr-master/drivers/ipm/Kconfig
zephyr-master/drivers/net/Kconfig (has BSD-3-Clause text, --> Apache-2.0? )
zephyr-master/drivers/serial/Kconfig.altera_jtag
zephyr-master/drivers/serial/Kconfig.cc32xx
zephyr-master/drivers/serial/Kconfig.esp32
zephyr-master/drivers/serial/Kconfig.miv
zephyr-master/drivers/serial/Kconfig.msp432p4xx
zephyr-master/drivers/serial/Kconfig.native_posix
zephyr-master/drivers/serial/Kconfig.ns16550
zephyr-master/drivers/serial/Kconfig.nsim
zephyr-master/drivers/serial/Kconfig.qmsi
zephyr-master/drivers/serial/Kconfig.stellaris
zephyr-master/drivers/timer/legacy_api.h

Expected behavior
Expect to see comment with
SPDX-License-Identifier: Apache-2.0
in each of the files, like the rest of the repository.

In cases where the file is not an Apache-2.0 one,
the appropriate SPDX Licence Identifier should be used.

Impact
Showstopper. Licensing needs to be clean and comprehensive before LTS is released, as this will impact all projects trying to generate an accurate SBOM, as well as impact CII Gold badge status.

Environment (please complete the following information):
Version used: 1.14-rc3