Coffee Lake CPU on Kaby Lake PCH is possible.
It does exist, and it does work.

The only missed case was this warning here noticed
by Wendy who could get one system with this configuration
and reported the issue for us:

Hardware Configuration
Board ID KBL S DDR4 UDIMM EV CRB
Processor    Intel® Processor code named Coffee Lake S, (6+2), 6 cores 12 threads, GT2, A0 (Internal) (QNJ4)

[ 3.220585] WARNING: CPU: 10 PID: 206 at drivers/gpu/drm/i915/i915_drv.c:340 i915_driver_load+0x1210/0x1660 [i915]
[ 3.221312] Modules linked in: hid_generic usbhid i915 i2c_algo_bit drm_kms_helper e1000e syscopyarea sysfillrect sysimgblt nvme fb_sys_fops ptp ahci i2c_hid drm pps_core nvme_core libahci wmi hid video
[ 3.222050] CPU: 10 PID: 206 Comm: systemd-udevd Not tainted 4.13.0-rc5-intel-next+ #1
[ 3.222706] Hardware name: Intel Corporation Kabylake Client platform/KBL S DDR4 UDIMM EV CRB, BIOS KBLSE2R1.R00.X089.P00.1705051000 05/05/2017

Cc: Wendy Wang <wendy.wang@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170821235056.9015-1-rodrigo.vivi@intel.com