feat(fvp): introduce PLATFORM_TEST_EA_FFH config

feat(fvp): introduce PLATFORM_TEST_EA_FFH config

FVP currently does not have proper handler to do Firmware First Handling
(FFH) of lower EL External aborts and it ends up in EL3 panic.

To test the scenarios sensibly we need a proper handling when the FVP is
under test so that we do not change the default behavior.

Introduce PLATFORM_TEST_EA_FFH config which will be enabled in CI
scripts and implement a proper handling for Sync EA and SErrors from
lower EL.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib130154206b17f72c49c9f07de2d92f35a97ab0b