ARM: imx: remove unused imx3 pm definitions

Remove the defintion of mx3_cpu_lp_set(), this function is not
implemented anywhere. Remove then mx3_cpu_pwr_mode enum as well,
it was used only as parameter of mx3_cpu_lp_set().

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>