powerpc: Implement arch_get_random_long/int() for powernv

Add the plumbing to implement arch_get_random_long/int(). It didn't seem
worth adding an extra ppc_md hook for int, so we reuse the one for long.

Add an implementation for powernv based on the hwrng found in power7+
systems. We whiten the output of the hwrng, and the result passes all
the dieharder tests.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/machdep.h

@@ -263,6 +263,10 @@ struct machdep_calls {
 	ssize_t (*cpu_probe)(const char *, size_t);
 	ssize_t (*cpu_release)(const char *, size_t);
 #endif
+
+#ifdef CONFIG_ARCH_RANDOM
+	int (*get_random_long)(unsigned long *v);
+#endif
 };
 
 extern void e500_idle(void);