regulator: tps65218: Add OF dependency

This is a DT-only driver, so make it depend on OF and remove of_match_ptr in
the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>