tc2/tfa: Update build flag as recommended by TF-A

* Change ENABLE_SVE_FOR_NS build flag to 2 as advised here:-
  https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=67265f2f6d7604147080033a1c99150e9a020f28


  In summary, TF-A now supports 3 states for NS feature flags. 0 for disabled, 1 for always enabled
  and 2 for dynamically deciding. TF-A recommends to use option 2 for virtual platforms. Hence changing this flag
  to 2 for FVP and keeping it as 1 for fpga.

Signed-off-by: Tintu Thomas <tintu.thomas@arm.com>