dt-bindings: mhi: add support to override local transfer ring length

Some MHI channels such as RSC channels require different local ring
length than transfer ring length. This devicetree property allows
users to override local transfer ring length when needed.

CRs-Fixed: 2361266
Change-Id: I7ef961cb81f1c0dae8899d97347c0f32afba2966
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>