dts: enable uart3 for frdm_k64f

Building `smp_svr` for this board results in a build error due to
undefined symbols `DT_NXP_KINETIS_UART_4006D000_*` which are related to
uart3 DTS symbols not being generated. This marks uart3's status as OK.

Signed-off-by: Fabio Utzig <utzig@apache.org>