boards: arm: Create unified fvp_base_revc_2xaem board structure

Create unified board configuration to replace verbose board naming
with shorter hierarchical structure. This board works with the new
unified fvp_aem SOC series and supports the v8a architecture variant.

This enables board targets like:
- fvp_base_revc_2xaem/v8a (basic ARMv8-A)
- fvp_base_revc_2xaem/v8a/smp (ARMv8-A SMP)
- fvp_base_revc_2xaem/v8a/smp/ns (ARMv8-A SMP non-secure)

This structure replaces the existing overly verbose board designations
with a cleaner, more maintainable organization and provides a foundation
for adding v9a variants.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
29 files changed