target/arm: Add state for the ARMv8.3-PAuth extension

target/arm: Add state for the ARMv8.3-PAuth extension

Add storage space for the 5 encryption keys.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190108223129.5570-2-richard.henderson@linaro.org
[PMM: use 0xf rather than -1 in FIELD_DP64() expressions to
 avoid clang warnings about implicit truncation from int to
 bitfield changing the value]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>