dt-bindings: drm: msm-lease: add empty resource support

Add support to msm-lease with empty resource defined in devicetree.
If a card is defined with empty resource, all resources that are
not leased by other cards will be leased to this card.

Change-Id: I60550bbe69bb71f8bfce9ceac959678b0bb6df4d
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>