Documentation / MTD: Rename the intel-spi driver

Since the driver is renamed (and moved) update the BIOS upgrade guide
accordingly from intel-spi to spi-intel. Keep the guide under MTD
documentation because this is pretty much still about MTD and SPI-NOR.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mauro Lima <mauro.lima@eclypsium.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20220209122706.42439-4-mika.westerberg@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

docs: add SPDX tags to new index files

All those new files I added are under GPL v2.0 license.

Add the corresponding SPDX headers to them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

docs: mtd: move it to the driver-api book

While I was tempted to move it to admin-guide, as some docs
there are more userspace-faced, there are some very technical
discussions about memory error correction code from the Kernel
implementer's PoV. So, let's place it inside the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>