arm: mach-k3: security: Remove certificate if detected on GP device

If the device is a GP and we detect a signing certificate then remove it.
It would fail to authenticate otherwise as the device is GP and has no
secure authentication services in SYSFW.

This shouldn't happen often as trying to boot signed images on GP devices
doesn't make much sense, but if we run into a signed image we should at
least try to ignore the certificate and boot the image anyway. This could
help with users of GP devices who only have HS images available.

If this does happen, print a nice big warning.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>