mfd: tps6586x: Add version detection

Use the VERSIONCRC to determine the exact device version. According to
the datasheet this register can be used as device identifier. The
identification is needed since some tps6586x regulators use a different
voltage table.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>