power: reset: ltc2952: remove global variable poweroff_panic

As per Documentation/CodingStyle ch.4, we should keep global variables
to a mininum. Move the panic state into the driver data, regardless of
whether panic is a system state or not.

This removes the need for the custom _init and _exit functions, so
replace them with a call to the module_platform_driver() macro.

Signed-off-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>