feat(sve): support full SVE vector length

Currently the SVE code hard codes a maximum vector length of 512 bits
when configuring SVE rather than the architecture supported maximum.
While this is fine for current physical implementations the architecture
allows for vector lengths up to 2048 bits and emulated implementations
generally allow any length up to this maximum.

Since there may be system specific reasons to limit the maximum vector
length make the limit configurable, defaulting to the architecture
maximum. The default should be suitable for most implementations since
the hardware will limit the actual vector length selected to what is
physically supported in the system.

Signed-off-by: Mark Brown <broonie@kernel.org>
Change-Id: I22c32c98a81c0cf9562411189d8a610a5b61ca12