boards: arm: efr32_thunderboard: add 'zephyr,bt-c2h-uart'

Include 'zephyr,bt-c2h-uart' in 'chosen' node to make it possible to use
HCI raw mode over UART. This was tested with 'efr32bg22_brd4184b' board
running 'hci_uart' sample with Linux machine as a host (kernel 6.2).

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>