arm64/sve: Kconfig update and conditional compilation support

This patch adds CONFIG_ARM64_SVE to control building of SVE support
into the kernel, and adds a stub predicate system_supports_sve() to
control conditional compilation and runtime SVE support.

system_supports_sve() just returns false for now: it will be
replaced with a non-trivial implementation in a later patch, once
SVE support is complete enough to be enabled safely.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>