dma-buf-map: Add memcpy and pointer-increment interfaces

To do framebuffer updates, one needs memcpy from system memory and a
pointer-increment function. Add both interfaces with documentation.

v5:
	* include <linux/string.h> to build on sparc64 (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103093015.1063-10-tzimmermann@suse.de