soc: qcom: qcom_ipcc: clear interrupt first

IPCC should simulate the edge interrupt handling provided by GIC. Clear
the interrupt before callng the ISR to prevent missing interrupts fired
during the ISR execution.

Change-Id: I617c74cb8cd50b307dab8414fb08fe9c59c23880
Signed-off-by: Chris Lew <clew@codeaurora.org>