ArmPlatformPkg/PrePeiCore: enable VFP at startup

ArmPlatformPkg/PrePeiCore: enable VFP at startup

While the alternative PEI-less SEC implementation in PrePi already
takes the EnableVFP PCD into account, the PrePeiCore code does not,
and so we may end up triggering synchronous exception when code
attempts to use FP or SIMD registers, which is permitted on AARCH64
by the spec.

So enable the VFP as early as feasible if the associated PCD is set.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>