[ARM] mm 3: separate out supersection mappings, avoid for <4GB

Catalin Marinas at ARM Ltd says:
> The CPU architects in ARM intended supersections only as a way to map
> addresses >= 4GB. Supersections are not mandated by the architecture
> and there is no easy way to detect their hardware support at run-time
> (other than checking for a specific core). From the analysis done in
> ARM, there wasn't a clear performance gain by using supersections
> rather than sections (no significant improvement in the TLB misses).

Therefore, we should avoid using supersections unless there's a real
need (iow, we're mapping addresses >= 4GB).

This means that we can simplify create_mapping() a bit since we will
only use supersection mappings for addresses >= 4GB, which means that
the physical, virtual and length must be multiples of the supersection
mapping size.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>