boards: nrf5340dk_nrf5340_cpunet: Disable BT_ECC support

Disable BT_ECC support in network core to reduce RAM usage.
ECC implementation can instead be supported in the Host
running in the application core.

This is also to keep hci_uart (nRF52 Series) and hci_rpmsg
(nRF53 Series) have the same support features. ECC is not
enabled in hci_uart sample.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>