spi: Extend the core to ease integration of SPI memory controllers ...

Sync with Linux v5.1-rc1.

This is the barebox adoption of the commit

  commit c36ff26
  Author: Boris Brezillon <boris.brezillon@bootlin.com>
  Date:   Thu Apr 26 18:18:14 2018 +0200

      spi: Extend the core to ease integration of SPI memory controllers

      Some controllers are exposing high-level interfaces to access various
      kind of SPI memories. Unfortunately they do not fit in the current
      spi_controller model and usually have drivers placed in
      drivers/mtd/spi-nor which are only supporting SPI NORs and not SPI
      memories in general.

      This is an attempt at defining a SPI memory interface which works for
      all kinds of SPI memories (NORs, NANDs, SRAMs).

      Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
      Reviewed-by: Frieder Schrempf <frieder.schrempf@exceet.de>
      Tested-by: Frieder Schrempf <frieder.schrempf@exceet.de>
      Signed-off-by: Mark Brown <broonie@kernel.org>

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>