mfd: twl6040: Select i2c fast mode as default with regmap patch

All boards using twl6040 configures the i2c bus to 400KHz. While twl6040's
defaults to normal mode (100KHz). So far twl6040 has no problem with i2c
communication in this configuration it is safer to select fast i2c mode.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>