boards: thingy53_nrf5340: Do not forward FEM SPI pins to cpunet

The SPI cannot be shared between the cores. The MPSL running on
the network core does not support SPI comminucation yet, so the
application core sets FEM's CS to inactive state.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>