remoteproc: qcom: pas: Carry PAS metadata context

Starting with Qualcomm SM8450 the metadata object shared with the secure
world during authentication and booting of a remoteproc needs to be
alive from init_image() until auth_and_reset().

Use the newly introduced "PAS metadata context" object to track this
context from load until the firmware has been booted.

In the even that load is performed but the process for some reason
doesn't reach auth_and_reset the unprepare callback is used to clean up
the allocated memory.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>