arm64: Documentation: cpu-feature-registers: Remove RES0 fields

Remove the invisible RES0 field entries from the table, listing
fields in CPU ID feature registers, as :
 1) We are only interested in the user visible fields.
 2) The field description may not be up-to-date, as the
    field could be assigned a new meaning.
 3) We already explain the rules of the fields which are not
    visible.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>