msm: pil-gss: Force into low power mode explicitly

PIL core code already calls shutdown on all processor during late
init, but we would like to remove that code. Therefore force GSS
into low power mode by explicitly calling the shutdown op during
driver probe. This achieves the same goal without requiring the
PIL core to call shutdown on all processors.

Change-Id: I13f0d2d6544d9e8d7668272e1059b27a1767ea8e
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>