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flash/nor: add DesignWare SPI controller driver

Driver for DesignWare SPI controller, found on many SoCs (see compatible
list in Linux device tree bindings
Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml). This
implementation only supports MIPS as it was the only one available for the
tests, however, adding support for other architectures should require only
few adjustments. Driver relies on flash/nor/spi.h to find Flash chip info.
Driver internal functions support 24bit addressing mode, but due to
limitations of flash/nor/spi.h, it is not used. The repoted writing speed
is about 60kb/s.
Lint, sanitizer and valgraind repoted warnings were not related to the
driver.

Change-Id: Id3df5626ab88055f034f74f274823051dedefeb1
Signed-off-by: Sergey Matsievskiy <[email protected]>