arm64/sve: Document firmware support requirements in Kconfig

Use of SVE by EL2 and below requires explicit support in the
firmware.  There is no means to hide the presence of SVE from EL2,
so a kernel configured with CONFIG_ARM64_SVE=y will typically not
work correctly on SVE capable hardware unless the firmware does
include the appropriate support.

This is not expected to pose a problem in the wild, since platform
integrators are responsible for ensuring that they ship up-to-date
firmware to support their hardware.  However, developers may hit
the issue when using mismatched compoments.

In order to draw attention to the issue and how to solve it, this
patch adds some Kconfig text giving a brief explanation and details
of compatible firmware versions.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>