msm: ADSPRPC: Support for RPC to multiple DSP processors

Open separate channels to each DSP processor based on the
file descriptor to support remote procedure calls to
multiple DSP processors.

Change-Id: I9c51d15b38eef67ccc38fc6dd0867d8256fbbdf6
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>