configs: Create minimal vexpress_fvp_defconfig

The vexpress64 board family now relies on OF_CONTROL and
OF_HAS_PRIOR_STAGE, so platform-specific configuration requirements
are minimal.

The vexpress_aemv8a_semi_defconfig file defines many flags that are not
needed for a minimal boot, such as flash memory configuration.

Therefore create vexpress_fvp_defconfig which contains the minimum
configuration required to boot on an Arm v8a FVP.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>