hwmon: (ina2xx) Fix build issue by selecting REGMAP_I2C

Since a0de56c8 ("hwmon: (ina2xx) convert driver to using regmap")
the driver requires REGMAP_I2C to build.  Select it by default
in Kconfig.

Reported-by: Guo Chunrong <B40290@freescale.com>
Cc: Marc Titinger <mtitinger@baylibre.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Fixes: a0de56c8 ("hwmon: (ina2xx) convert driver to using regmap")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>