HACK: dt-bindings: misc: Add ti,dma_buf_phys bindig doc

Driver allowing user-space attaching of dmabuf fd's returning CPU
physical addresses. The reasoning for making this act like a regular
device described by DT is so the virtual device that binds the buffer
can be made to act as if it is out on a bus or behind an IOMMU, for
example.

Add this binding here. This is a hack and marked as such.

Signed-off-by: Gowtham Tammana <g-tammana@ti.com>