serial: dts: remove !HAS_DTS related Kconfig

As we now only support DTS for ARM based SoCs we can remove any
associated !HAS_DTS bits in Kconfig.  The Nordic NRF5 serial driver and
the ARM CMSDK APB serial drivers had Kconfig bits related to !HAS_DTS
builds.

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