boards: mec15xxevb_assy6853: Disable KSCAN to avoid test case issues

This change addresses issue# 21756 where the expected thread
count does not match because kscan module is enabled in this board.
The Microchip kscan driver is contributing to the total thread count,
therefore interfering with the expected test result.

Signed-off-by: Francisco Munoz <francisco.munoz.ruiz@intel.com>