Add I2C4 clock support for i.MX6SX. Since we use runtime check, but not macro, we need to remove `#ifdef ..` in crm_regs.h, or gcc will fail to compile the code succesfully. Making the macros only for i.MX6SX open to other i.MX6x maybe not a good choice, but we have runtime check. Signed-off-by: Peng Fan <Peng.Fan@freescale.com>