arm: dts: Add bindings for Cortex-A78

Add CPU bindings for the Cortex-A78 to enable reading CPU device tree
properties in Kconfig. This is required to correctly configure and
use CPU-specific settings based on the device tree.

Signed-off-by: Appana Durga Kedareswara rao <appana.durga.kedareswara.rao@amd.com>