ARM: OMAP2+: PM: only register TWL with voltage layer when device is present

Current code registers voltage layer details for TWL PMIC even when a TWL
has not been registered.  Fix this to only register the TWL with voltage
layer when the TWL PMIC is initialized by board-level code.

Tested-by: Abhilash Koyamangalath <abhilash.kv@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>