mhi: core: put mhi channel, events, and device into a child node

Fix devicetree warning "duplicate unit-address" between MHI channel
nodes, MHI event nodes, and MHI device node by moving each group into
child nodes.

CRs-Fixed: 2313588
Change-Id: I9824025f37f65c34103365ef96e7c23367f5a16e
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>