msm: pil: Allow verify_blob() to be optional

Most pil drivers don't verify blobs until the peripheral is
brought out of reset. The blobs are verified during the
auth_and_reset() callback instead. Make the verify_blob()
callback optional so that drivers don't have to provide dummy
functions.

Change-Id: I183593a224d51db04f3a52a192adddd632ad2306
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>