manifest: Update nRF hw models to latest

Update the HW models module to:
b84bd7314a239f818e78f6927f5673247816df53

Including the following:
b84bd73 UART PTY backend: Correct a few comments
f945d62 RADIO 54: Fix reset value of TIMING register
9fd4133 MDK nrfx_erratas.h: Ensure backwards compatibility with old
        nrfx versions
85061b1 MDK: Provide empty replacement for nrf_erratas.h

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