boards: Document watchdog driver support on k64, kw2xd boards

Updates documentation for the frdm_k64f, hexiwear_k64, and usb_kw24d512
boards to reflect that they now support a watchdog driver. Updates the
board yamls similarly for sanitycheck.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>
6 files changed