boards: mps4: Add initial support for corstone315

What is changed?
  - Add initial support for the MPS4 Corstone-315 FVP platform, including
    board and SoC definitions.The qualifier to build/run application
    with board mps4/corstone315 is
    `mps4/corstone315/fvp` for secure and
    `mps3/corstone315/fvp/ns` for non-secure.
  - FVP testing with corstone315 uses the ARM FVP
    `FVP_Corstone_SSE-315`.

Why do we need this change?
  - This enables FVP support for corstone315.
  - A separate FVP variant was added for corstone315 as the TFM board
    used for non-secure variant differs for FPGA and FVP.
    TFM board `arm/mps4/corstone315` support is present but no FVP support
    yet. We can test this by building TF-M with
    -DTFM_PLATFORM=arm/mps4/corstone315 and then lauching FVP:
    FVP_Corstone_SSE-315 --data "bl1_1.bin"@0x11000000
      --data "cm_provisioning_bundle.bin"@0x12024000
      --data "dm_provisioning_bundle.bin"@0x1202aa00
      --data "bl2_signed.bin"@0x12031400
      --data "tfm_s_ns_signed.bin"@0x38000000

Signed-off-by: Shaunak saha <ssaha@tsavoritesi.com>