clocksource/drivers/exynos_mct: Use container_of() instead of this_cpu_ptr()

Since evt structure is embedded in per-CPU mevt structure it's
definitely faster to use container_of() to get access to mevt
if we have evt (for example as incoming function argument) instead
of more expensive approach with this_cpu_ptr(&percpu_mct_tick).
this_cpu_ptr() on per-CPU mevt structure leads to access to cp15
to get cpu id and arithmetic operations.
Container_of() is cheaper since it's just one asm instruction.
This should work if used evt pointer is correct and owned by
local mevt structure.

For example, before this patch set_state_shutdown() looks like:

 4a4:	e92d4010 	push	{r4, lr}
 4a8:	e3004000 	movw	r4, #0
 4ac:	ebfffffe 	bl	0 <debug_smp_processor_id>
 4b0:	e3003000 	movw	r3, #0
 4b4:	e3404000 	movt	r4, #0
 4b8:	e3403000 	movt	r3, #0
 4bc:	e7933100 	ldr	r3, [r3, r0, lsl #2]
 4c0:	e0844003 	add	r4, r4, r3
 4c4:	e59400c0 	ldr	r0, [r4, #192]	; 0xc0
 4c8:	ebffffd4 	bl	420 <exynos4_mct_tick_stop.isra.1>
 4cc:	e3a00000 	mov	r0, #0
 4d0:	e8bd8010 	pop	{r4, pc}

With this patch:

 4a4:	e92d4010 	push	{r4, lr}
 4a8:	e59000c0 	ldr	r0, [r0, #192]	; 0xc0
 4ac:	ebffffdb 	bl	420 <exynos4_mct_tick_stop.isra.1>
 4b0:	e3a00000 	mov	r0, #0
 4b4:	e8bd8010 	pop	{r4, pc}

Also, for me size of exynos_mct.o decreased from 84588 bytes
to 83956.

Signed-off-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>