ARM: multi_v7_defconfig: update XILINX_VDMA

Commit fde57a7c4474
("dmaengine: xilinx: Rename driver and config")

renamed config XILINX_VDMA to config XILINX_DMA
Update defconfig accordingly.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>