mfd: tps65218: add version check to the PMIC probe

Version information will be needed to handle some error cases under the
regulator driver, so store the information once during MFD probe.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>