uboot-imx

dtoc: Rename is_phandle() and adjust it to return more detail

Update this function to return more detail about a property that contains
phandles. This will allow (in a future commit) more accurate handling of
these properties.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>