ARM: OMAP2+: voltage: Remove some unused functions

Removes some functions that are not used anywhere:
omap_change_voltscale_method() voltdm_add_pwrdm()
voltdm_for_each() voltdm_for_each_pwrdm()

And remove define VOLTSCALE_VPFORCEUPDATE and VOLTSCALE_VCBYPASS

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Tony Lindgren <tony@atomide.com>