powerpc/64: Move MMU backend selection out of platform code

We move it into early_mmu_init() based on firmware features. For PS3,
we have to move the setting of these into early_init_devtree().

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

arch/powerpc/include/asm/ps3.h

@@ -526,4 +526,6 @@ void ps3_sync_irq(int node);
 u32 ps3_get_hw_thread_id(int cpu);
 u64 ps3_get_spe_id(void *arg);
 
+void ps3_early_mm_init(void);
+
 #endif