powerpc: Add AT_HWCAP2 to indicate V.CRYPTO category support

The Vector Crypto category instructions are supported by current POWER8
chips, advertise them to userspace using a specific bit to properly
differentiate with chips of the same architecture level that might not
have them.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.10+]