mtd: spi-nor / spi / MFD: Convert intel-spi to SPI MEM

Merge series from Mika Westerberg <mika.westerberg@linux.intel.com>:

Based on discussion on the patch I sent some time ago here:

  http://lists.infradead.org/pipermail/linux-mtd/2021-June/086867.html

it turns out that the preferred way to deal with the SPI flash controller
drivers is through SPI MEM which is part of Linux SPI subsystem.

This series does that for the intel-spi driver. This also renames the
driver to follow the convention used in the SPI subsystem. The first patch
improves the write protection handling to be slightly more safer. The
following two patches do the conversion itself. Note the Intel SPI flash
controller only allows commands such as read, write and so on and it
internally uses whatever addressing etc. it figured from the SFDP on the
flash device.

base-commit: e783362e