drivers: spi: add Data Fusion Subsystem SPI driver

Introduce DesignWare ARC Data Fusion IP Subsystem(DFSS) SPI
driver for ARC boards, i.e. EMSDP, which uses DW SPI to controll
SPI-Flash and DFSS SPI to connect external devices. Both drivers
share most source code, but DFSS uses ARC auxiliary registers.
Move FIFO depth setting to device tree.

Signed-off-by: Siyuan Cheng <siyuanc@synopsys.com>