crypto: ixp4xx - convert to platform driver

The ixp4xx_crypto driver traditionally registers a bare platform
device without attaching it to a driver, and detects the hardware
at module init time by reading an SoC specific hardware register.

Change this to the conventional method of registering the platform
device from the platform code itself when the device is present,
turning the module_init/module_exit functions into probe/release
driver callbacks.

This enables compile-testing as well as potentially having ixp4xx
coexist with other ARMv5 platforms in the same kernel in the future.

Cc: Corentin Labbe <clabbe@baylibre.com>
Tested-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>