ARM: dts: msm: Reserve MHI EV ring 0 for ctrl events for mdm9x55

In order to avoid latency of processing control
events when processing other events, reserve event
ring 0 only for ctrl events.

CRs-Fixed: 1081654
Change-Id: Ic9b2a4ee925569986abb46ce8c4688d539fd4d41
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>