ARM: OMAP2+: cm33xx.c: Remove some unused functions

Removes some functions that are not used anywhere:
am33xx_cm_read_reg_bits() am33xx_cm_clear_reg_bits() am33xx_cm_set_reg_bits()

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>