ARM: OMAP2+: Remove legacy PM init

This is no longer needed when booted with device tree.
And let's replace cpu_is with soc_is for the PM code to
avoid confusion, they do the same thing.

Note that omap_pmic_late_init() now just calls
omap3_twl_init() and omap4_twl_init() to initialize the
voltage layer so we can remove the remaining references
to twl-common code and remove it in the following patch.

Signed-off-by: Tony Lindgren <tony@atomide.com>