spi: Updates for v4.6

Not the biggest set of changes for SPI but a bit of a pickup in activity
on the core:

 - Support for memory mapped read from flash devices via a SPI
   controller.
 - The beginnings of a message rewriting framework in the core which
   should in time allow us to support transforming messages to work
   around the limits of controllers or optimise the performance for
   controllers transparently to calling drivers.
 - Updates to the PXA2xx, the main functional change being to improve
   the ACPI support.
 - A new driver for the Analog Devices AXI SPI engine.
Merge remote-tracking branches 'spi/topic/res', 'spi/topic/rockchip', 'spi/topic/sh', 'spi/topic/ti-qspi' and 'spi/topic/xilinx' into spi-next