samples: coaps_server: Fix platforms to build coap_server test on

We incorrectly had an arch_whitelist constraint set to 'qemu_x86' that
should have been a platform_whitelist, so fix that and add arduino_101
frdm_k64f to the platform_whitelist as targets that we can test this on.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>