boards: arm: Clarify J-Link OpenSDA firmware usage for frdm_k64f

Updates the frdm_k64f board documentation to clarify that the J-Link
OpenSDA firmware version depends on the DAPLink bootloader version.

Signed-off-by: Derek Snell <derek.snell@nxp.com>
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>