system_ctlreg_modify
void system_ctlreg_modify(unsigned int cr, unsigned long data, int request);
system_ctlreg_modify(cr, bit, CTLREG_SET_BIT);
system_ctlreg_modify(cr, bit, CTLREG_CLEAR_BIT);
system_ctlreg_modify(cr, reg->val, CTLREG_LOAD);
EXPORT_SYMBOL(system_ctlreg_modify);