part: Export part_driver_lookup_type for external use

part: Export part_driver_lookup_type for external use

Make part_driver_lookup_type non-static so it can be used outside
part.c. This allows external callers to determine the appropriate
partition driver for a block device, enabling more flexible handling of
partition types.

Add a prototype and kernel-doc comment in part.h to document the
function contract. Provide a stub inline implementation returning NULL
when partition support is disabled, ensuring build consistency across
configurations.

Signed-off-by: Javier Tia <javier.tia@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>