powerpc/mm: Abstraction for switch_mmu_context()

How we switch MMU context differs between hash and radix. For hash we
need to switch the SLB details and for radix we need to switch the PID
SPR.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/cpufreq/pmac32-cpufreq.c

@@ -298,7 +298,7 @@ static int pmu_set_cpu_speed(int low_speed)
  		_set_L3CR(save_l3cr);
 
 	/* Restore userland MMU context */
-	switch_mmu_context(NULL, current->active_mm);
+	switch_mmu_context(NULL, current->active_mm, NULL);
 
 #ifdef DEBUG_FREQ
 	printk(KERN_DEBUG "HID1, after: %x\n", mfspr(SPRN_HID1));