spi: Updates for v4.21

The main thing this release has been a lot of work on the integration
with SPI NOR flashes, there's been some specific support for a while for
controller features designed to make them perform better but it's not
worked out as well as hoped so the interface has been redesigned in a
way that will hopefully do better - it's already been adopted by a
number of additional controllers so things are looking good.  Otherwise
most of the work has been driver specific:

 - Support for better integration with NOR flashes from Boris Brezillon
   and Yogesh Narayan Gaur plus usage of it in several drivers.
 - A big cleanup of the Rockchip driver from Emil Renner Berthing.
 - Lots of performance improvements for bcm2835 from Lukas Wunner.
 - Slave mode support for pxa2xx from Lubomir Rintel.
 - Support for Macronix MXIC, Mediatek MT7629 and MT8183, NPCM PSPI,
   and Renesas r8a77470.
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Merge remote-tracking branches 'spi/topic/mem' and 'spi/topic/mtd' into spi-next