hwmon: (ina2xx) convert driver to using regmap

Any sysfs "show" read access from the client app will result in reading
all registers (8 with ina226). Depending on the host this can limit the
best achievable read rate.

This changeset allows for individual register accesses through regmap.

Tested with BeagleBone Black (Baylibre-ACME) and ina226.
Signed-off-by: Marc Titinger <mtitinger@baylibre.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>