Could you please clarify which register you need to configure? Are you using Cypress SampleSW?
For example, if MCU enters sleep mode, how does the register controlling IO be configured?
Do you mean the HOLDIO_PDx and PSS port control register?
If yes, you can configure register SYSC0_SPECFGR. Here is an example:
// Select PSS IO hold and high-z
SYSC0_PROTKEYR = 0x5cacce55 ; // Unlock
SYSC0_SPECFGR = 0x01000200; // HOLDIO_PD2=1, PSSPADCTRL=0, IO3RSTC=0, IO35RSTC=0, EXVRSTCNT=1
For S6J3400, please kindly refer to the register SYSC0_SPECFGR description marked with LP_type in S6J3400 Hardware Manual Platform part (002-07884 Rev. *D).