mfd: tps65912: Move regmap config into core driver

When building with extra warnings enabled, most files including
linux/mfd/tps65912.h warn about a static variable defined in the
header:

include/linux/mfd/tps65912.h:331:35: warning: 'tps65912_regmap_config' defined but not used [-Wunused-const-variable=]

We also duplicate the data structure between the i2c and spi front-end
drivers. Moving it into the driver code avoids the warning and
the duplication.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>