ARM: OMAP2+: powerdomain.c: Remove some unused functions

Removes some functions that are not used anywhere:
pwrdm_get_voltdm() pwrdm_for_each_clkdm() pwrdm_del_clkdm()

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

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>