ARM: ebsa110: Properly override I/O accessors

In order to override accessors properly they must be #define'd so that
subsequent generic headers (the one for ARM and finally the architecture
independent one) can properly detect it.

While at it, make all accessors use volatile void __iomem * to avoid a
slew of build warnings.

Signed-off-by: Thierry Reding <treding@nvidia.com>