manifest: Update nrf hw models to latest

* Update the HW models module to
6b6ae3652c92c95edd945dce6ad9ef9892aab89b

Including the following:
* 6b6ae36 UART: Minor optimization
* afe84c1 fake timer: Minor speed optimization for nrf52
* a5a4dfd RTC: Fix counter value when CLEAR after STOP

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