mfd: tps65217: Introduce dependency on CONFIG_OF

Currently the driver boots only via device tree hence add a
dependency on CONFIG_OF. This leaves with a bunch of unused code
so clean that up. This patch also makes use of probe_new function
in place of the probe function so as to avoid passing i2c_device_id.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>