crypto: tegra - use kernel entropy instead of ad-hoc

The way I read the Tegra AES RNG is that it has a homebrew
algorithm for initializing the 128bit RNG using timespec and
the unique chip ID. This looks like reinventing the (square)
wheel, instead just grab 128bits from the kernel entropy pool
where the time and (after another patch) chip unique ID is
already mixed in.

Incidentally this also gets rid of a rather ugly
cross-dependence on the machine using an extern declaration.

Cc: Varun Wadekar <vwadekar@nvidia.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>