mfd: twl6040: Remove support for legacy (pdata) mode

TWL6040 is used only with OMAP4/5 SoCs and they can only boot in in DT mode.
The support for pdata/legacy boot can be removed.

Add TODO comment to the header file that all pdata struct can be removed in
the next merge window (after the sub driver updates are in).

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