dt-bindings: mhi: net_dev: add disable chain-skb property

Not all MHI net devices use chaining. Update devicetree documentation
to reflect new option to disable chaining if required.

CRs-Fixed: 2361266
Change-Id: Ie78838fcb1f4ac69d2615385fba3f2e3abf76087
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>