cmake: dts.cmake: Use SOC_PATH to find soc/dts.fixup

Per arch the path to the SOC dir that we would find dts.fixup varies
between being ${CONFIG_SOC_FAMILY}/${CONFIG_SOC_SERIES} and just
${CONFIG_SOC}.  The ${SOC_PATH} var handles defining this correct so
use it instead when we define DTS_SOC_FIXUP_FILE.

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