samples: net: lwm2m_client: add "ok" and baud rate to k64f overlay

During the modem overlay move from the K64F board directory to the
LwM2M sample code, 2 DTS entries were dropped:
current-speed = <115200>;
status = "ok";

Without these entries the DTS defines for UART2 and CURRENT_SPEED
are missing from the generated_dts_board.h file.

Signed-off-by: Michael Scott <mike@foundries.io>