mx6: crm_regs: Fix MXC_CCM_CLPCR_WB_PER_AT_LPM definition

According to the Reference Manual the 'wb_per_at_lpm' field of register
CCM_CLPCR corresponds to bit 16 so fix its definition accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>