KVM: Documentation: Document arm64 core registers in detail

Since the the sizes of individual members of the core arm64
registers vary, the list of register encodings that make sense is
not a simple linear sequence.

To clarify which encodings to use, this patch adds a brief list
to the documentation.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>