SAM3,4,A5 DMA fixes; SAMA5 SPI driver now supports DMA transfers

arch/arm/src/sam34/chip/sam3u_dmac.h

arch/arm/src/sam34/sam3u_dmac.c

arch/arm/src/sam34/sam_dmac.h

arch/arm/src/sama5/Kconfig

arch/arm/src/sama5/chip/sam_dmac.h

arch/arm/src/sama5/sam_dmac.c

arch/arm/src/sama5/sam_dmac.h

arch/arm/src/sama5/sam_hsmci.c

arch/arm/src/sama5/sam_spi.c

configs/sama5d3x-ek/README.txt

configs/sama5d3x-ek/src/sam_nsh.c

drivers/mtd/at25.c