spi: spi-gxp: Add support for HPE GXP SoCs

spi: spi-gxp: Add support for HPE GXP SoCs

The GXP supports 3 separate SPI interfaces to accommodate the system
flash, core flash, and other functions. The SPI engine supports variable
clock frequency, selectable 3-byte or 4-byte addressing and a
configurable x1, x2, and x4 command/address/data modes. The memory
buffer for reading and writing ranges between 256 bytes and 8KB. This
driver supports access to the core flash and bios part.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Link: https://lore.kernel.org/r/20220728161459.7738-2-nick.hawkins@hpe.com
Signed-off-by: Mark Brown <broonie@kernel.org>