samples: Bluetooth: hci_rpmsg: Fix RAM overflow building for DF feature

Fix RAM overflow building for direction finding feature on
nRF53 Series, by reducing thread stack sizes. And move
building DF plus CIS feature to hci_uart sample from
hci_rpmsg sample.

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