spi: gxp_spi: Add GXP SPI controller driver

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.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>