mhi: core: Enable NER support

Enable support for NER as part of the MHI REV E
specification. NER register will now be set to the
maximum number of channels supported by MHI.

Change-Id: Id868c04886af13cd34720c86a55ac64debc4ab31
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>