msm: mhi-dev: Add dev nodes for DIAG support on sdxlemur

Support dev node creation for DIAG_IN and DIAG_OUT channels
to enable DIAG traffic to go over PCIe.

Change-Id: Ib11958ee1ed1a40eb7c3bb719695231ad628d014
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>