spi: Updates for v3.20 The major highlight this release is a refactoring of the core to allow us to run synchronous transfers in the context of the caller when there is no contention for the bus. This improves performance in the very common case by eliminating context switches and reducing the number of hardware setup and teardown operations we need to perform. Other changes: - New drivers for DLN-2 USB-SPI adapter and ST SPI controllers. - A big round of cleanups, performance and feature improvements for the xilinx driver from Ricardo Ribalda Delgado. - A wide range of smaller cleanups, fixes and feature improvements throughout the subsystem.