spi: pic32-sqi: add SPI driver for PIC32 SQI controller.

This driver implements SPI master interface for Quad SPI
controller, specifically for accessing quad SPI flash.
It uses descriptor-based DMA transfer mode and supports
half-duplex communication for single, dual and quad SPI
transactions.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>