mfd: ti-lmu: Remove LM3532 backlight driver references

Remove the LM3532 backlight driver references from the ti-lmu
code as dedicated driver support is available.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Acked-for-MFD-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>