spmi: pmic-arb: Reserve a channel for debug port

Do not keep the channel reserved for debug port in the
ppid to apid mapping table. This is to avoid accessing
that particular channel during the read/write/irq operations.

Change-Id: I8f49d1d87978a5b68ea711d3e30606d72fd09f73
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>