manifest: Update nRF hw models to latest

Update the HW models module to:
968d55ff22579080466bf2f482596dd6e35361c6

Including the following:
968d55f 54LM20: Also build CRACEN RNG
2c6d49d CRACEN RNG: Add model of the new version of the IP
914b475 54LM20: Add first version
fb68cc6 grtc hal replacement: Remove pointless macro use
ef2f63e Fix wrong NRF_UARTE register reference
63a2e85 README: Clarify the models are not perfect, and correct links

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>