mfd: tps65912: Remove old driver in preparation for new driver

The old tps65912 driver is being replaced, delete old driver.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>