Bluetooth: samples: hci_uart: Update main stack size nrf5 boards

Update main stack size for nrf5 boards. This is to support an
alternative hci_driver that has a higher stack size usage in hci driver
open. Measured stack usage in this case to 808/1024.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>