boards: amd: versalnet_apu: Add SMP board variant support

Add a new board variant versalnet_apu/amd_versalnet_apu/smp to support
Symmetric Multi-Processing (SMP) configuration for AMD Versal NET APU.

This commit introduces:
- SMP board variant with 4-core configuration (CONFIG_MP_MAX_NUM_CPUS=4)
- PSCI support for CPU power management operations
- ARM64 40-bit virtual and physical address space support
- Non-secure world execution (EL1 NS) configuration
- Compatible with Xilinx ARM Trusted Firmware boot flow

The SMP variant enables out-of-box multicore support for applications
that require parallel processing capabilities on the Versal NET APU
platform.

Usage:
  west build -b versalnet_apu/amd_versalnet_apu/smp <application>

Boot Flow: Boot PDI -> TF-A -> Zephyr (SMP)

Files added:
- boards/amd/versalnet_apu/versalnet_apu_amd_versalnet_apu_smp.yaml
- boards/amd/versalnet_apu/versalnet_apu_amd_versalnet_apu_smp.dts
- boards/amd/versalnet_apu/versalnet_apu_amd_versalnet_apu_smp_defconfig

Files modified:
- boards/amd/versalnet_apu/board.yml (added smp variant)

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