arm64: Add new CPU_CORTEX_A78 configuration for Cortex-A78 support

Introduce a new Kconfig option CPU_CORTEX_A78 to enable support for the
Arm Cortex-A78 CPU architecture within Zephyr. This configuration can be
selected by boards or SoCs that utilize the Cortex-A78 core, enabling
architecture-specific features and optimizations as needed.

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