configs: qemu_arm_vexpress: enable vfp

In order to be abble to select BR2_ARM_FPU_VFPV3D16, BR2_ARM_ENABLE_VFP
must be selected first.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>