spmi: pmic-arb: optimize table lookups

The current driver uses a mix of radix tree and a fwd lookup
table to translate between apid and ppid. It is buggy and confusing.

Instead simply use a radix tree for v1 hardware and use the
forward lookup table for v2.

Change-Id: I74e1dc0c3dbbf2b311786af7d80a9d4810d510d7
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>