manifest: Update nrf hw models to latest

* Update the HW models module to
a08acc7d3a853f890df2bf82167c02ab2153ffe7

Including the following:
* a08acc7 UART(E): Duty cycle warnings due to Rx while Rx Off
* aa1f38d UART(E): FIFO backend Minor bugfix
* 8b0819f UART(E): Add Loopback backend
* bc648f2 UART(E): Update RTS pin level as soon as the conf is changed
* b39e448 UART(E): Detect receiver opening mid frame

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