drivers: uart_mcumgr: remove CONFIG_UART_MCUMGR_ON_DEV_NAME

Remove CONFIG_UART_MCUMGR_ON_DEV_NAME and use
DEVICE_DT_GET(DT_CHOSEN(zephyr_uart_mcumgr)).
Add usb.overlay, which contains chosen node and cdc-acm-uart node,
to smp_svr sample.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>