nxp-imx8: add kernel-cache configuration files for preempt-rt kernel

add kernel-cache configuration files for preempt-rt kernel

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

bsp/nxp-imx8/nxp-imx8-preempt-rt.scc

+define KMACHINE nxp-imx8
+define KTYPE preempt-rt
+define KARCH arm64
+
+include ktypes/preempt-rt
+branch nxp-imx8
+
+include nxp-imx8.scc