dmaengine: gpi: Dynamically assign gpii instance to clients

GPI dmaengine driver already has a list of supported gpii
instances. Also, all gpii instances are identical, support
all protocols, and operate on any serial engine. To reduce
configuration conflicts with clients, dynamically assign
next available gpii instance to clients.

CRs-Fixed: 2102800
Change-Id: I608f4a21bea372fe3641cf6c7dd0b4cdece44f2b
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>