debugfs: Export debugfs_create_str symbol

Needed by SCMI Raw mode support when compiled as a loadable module.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230118121426.492864-10-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>