target/arm: Fix PAuth sbox functions

In the PAC computation, sbox was applied over wrong bits.
As this is a 4-bit sbox, bit index should be incremented by 4 instead of 16.

Test vector from QARMA paper (https://eprint.iacr.org/2016/444.pdf) was
used to verify one computation of the pauth_computepac() function which
uses sbox2.

Launchpad: https://bugs.launchpad.net/bugs/1859713


Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Vincent DEHORS <vincent.dehors@smile.fr>
Signed-off-by: Adrien GRASSEIN <adrien.grassein@smile.fr>
Message-id: 20200116230809.19078-2-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>