msm: Support both forms of cache dumping

Currently, if dumping the caches on panic is enabled,
the caches will not be dumped from the bootloaders. This
means dumps will not be collected in watchdog cases.
Always set the magic numbers in IMEM for the bootloaders.
When a panic occurs, wipe the magic numbers so the
bootloaders don't overwrite the trustzone dumps.

Change-Id: I144c644b292e54504d1deb7dfcfa4e484ce251b9
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>