spi: spi-pic32: Add PIC32 SPI master driver

The PIC32 SPI driver is capable of performing SPI transfers
using PIO or external DMA engine. GPIO controlled /CS support
is made default in the driver for correct operation of the
controller. This can be enabled by adding "cs-gpios" property
of the SPI node in board dts file.

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