mfd: da9063: Fix revision handling to correctly select reg tables

The current implementation performs checking in the i2c_probe()
function of the variant_code but does this immediately after the
containing struct has been initialised as all zero. This means the
check for variant code will always default to using the BB tables
and will never select AD. The variant code is subsequently set
by device_init() and later used by the RTC so really it's a little
fortunate this mismatch works.

This update adds raw I2C read access functionality to read the chip
and variant/revision information (common to all revisions) so that
it can subsequently correctly choose the proper regmap tables for
real initialisation.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>