ARM: vexpress: remove automatic errata workaround selection

The vexpress Kconfig setup tries to be clever^Whelpful and selects some
errata workarounds for certain revisions of the Cortex-A9 and PL310,
which may be required depending on the core tile.

Since the mach-vexpress can support A5, A7 and A15 coretiles, let's
make errata workaround selection optional.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rob Herring <robherring2@gmail.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>