spi: pic32_spi: add SPI master driver for PIC32 SoC.

This driver implements SPI protocol in master mode to communicate
with the SPI device connected on SPI bus. It handles /CS explicitly
by controlling respective pin as gpio ('cs-gpios' property in dt node)
and uses PIO mode for SPI transaction. It is configurable based
on driver-model only.

Cc: Jagan Teki <jteki@openedev.com>
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>