asm: dma-mapping.h: Fix dma mapping functions

Subsystems such as USB expect dma_map_single() and dma_unmap_single() to
do dcache flush/invalidate operations as required. For example, see
see drivers/usb/gadget/udc/udc-core.c::usb_gadget_map_request().
Currently drivers do this locally, (see drivers/usb/dwc3/ep0.c,
drivers/mtd/nand/raw/denali.c etc..)
Update arch specific dma_map_single() and dma_unmap_single() APIs to do
cache flush/invalidate  operations, so that drivers need not implement
them locally.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rick Chen <rick@andestech.com>