There is CISS register at lpc23x/24xx, but I can't find it at lpc17xx_v07 user manual
Chapter 27: LPC17xx Real-Time Clock (RTC) and backup registers states that Periodic interrupts can be generated from increments of any field of the time registers and selected fractional second values.
But where is those selected fractional second values ?
... need to be directed to NXP!
This does, indeed, seem to be an error or omission in the NXP Document - so you need to contact NXP about it!
(it's nothing to do with Keil).