arm64: initialize spinlock for init_mm's context

ARM64 has defined the spinlock for init_mm's context, so need initialize
the spinlock structure; otherwise during the suspend flow it will dump
the info for spinlock's bad magic warning as below:

[   39.084394] Disabling non-boot CPUs ...
[   39.092871] BUG: spinlock bad magic on CPU#1, swapper/1/0
[   39.092896]  lock: init_mm+0x338/0x3e0, .magic: 00000000, .owner: <none>/-1, .owner_cpu: 0
[   39.092907] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G           O 3.10.33 #125
[   39.092912] Call trace:
[   39.092927] [<ffffffc000087e64>] dump_backtrace+0x0/0x16c
[   39.092934] [<ffffffc000087fe0>] show_stack+0x10/0x1c
[   39.092947] [<ffffffc000765334>] dump_stack+0x1c/0x28
[   39.092953] [<ffffffc0007653b8>] spin_dump+0x78/0x88
[   39.092960] [<ffffffc0007653ec>] spin_bug+0x24/0x34
[   39.092971] [<ffffffc000300a28>] do_raw_spin_lock+0x98/0x17c
[   39.092979] [<ffffffc00076cf08>] _raw_spin_lock_irqsave+0x4c/0x60
[   39.092990] [<ffffffc000094044>] set_mm_context+0x1c/0x6c
[   39.092996] [<ffffffc0000941c8>] __new_context+0x94/0x10c
[   39.093007] [<ffffffc0000d63d4>] idle_task_exit+0x104/0x1b0
[   39.093014] [<ffffffc00008d91c>] cpu_die+0x14/0x74
[   39.093021] [<ffffffc000084f74>] arch_cpu_idle_dead+0x8/0x14
[   39.093030] [<ffffffc0000e7f18>] cpu_startup_entry+0x1ec/0x258
[   39.093036] [<ffffffc00008d810>] secondary_start_kernel+0x114/0x124

Signed-off-by: Leo Yan <leoy@marvell.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>