clocksource: dw_apb_timer: Make CPU-affiliation being optional

Currently the DW APB Timer driver binds each clockevent timers to a
particular CPU. This isn't good for multiple reasons. First of all seeing
the device is placed on APB bus (which makes it accessible from any CPU
core), accessible over MMIO and having the DYNIRQ flag set we can be sure
that manually binding the timer to any CPU just isn't correct. By doing
so we just set an extra limitation on device usage. This also doesn't
reflect the device actual capability, since by setting the IRQ affinity
we can make it virtually local to any CPU. Secondly imagine if you had a
real CPU-local timer with the same rating and the same CPU-affinity.
In this case if DW APB timer was registered first, then due to the
clockevent framework tick-timer selection procedure we'll end up with the
real CPU-local timer being left unselected for clock-events tracking. But
on most of the platforms (MIPS/ARM/etc) such timers are normally embedded
into the CPU core and are accessible with much better performance then
devices placed on APB. For instance in MIPS architectures there is
r4k-timer, which is CPU-local, assigned with the same rating, and normally
its clockevent device is registered after the platform-specific one.

So in order to fix all of these issues let's make the DW APB Timer CPU
affinity being optional and deactivated by passing a negative CPU id,
which will effectively set the DW APB clockevent timer cpumask to
'cpu_possible_mask'.

Signed-off-by: Serge Semin <[e-mail]>
Cc: Alexey Malahov <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Paul Burton <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Cc: Alexandre Belloni <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/clocksource/dw_apb_timer.c

···
 /**
  * dw_apb_clockevent_init() - use an APB timer as a clock_event_device
  *
- * @cpu:	The CPU the events will be targeted at.
+ * @cpu:	The CPU the events will be targeted at or -1 if CPU affiliation
+ *		isn't required.
  * @name:	The name used for the timer and the IRQ for it.
  * @rating:	The rating to give the timer.
  * @base:	I/O base for the timer registers.
···
 	dw_ced->ced.max_delta_ticks = 0x7fffffff;
 	dw_ced->ced.min_delta_ns = clockevent_delta2ns(5000, &dw_ced->ced);
 	dw_ced->ced.min_delta_ticks = 5000;
-	dw_ced->ced.cpumask = cpumask_of(cpu);
+	dw_ced->ced.cpumask = cpu < 0 ? cpu_possible_mask : cpumask_of(cpu);
 	dw_ced->ced.features = CLOCK_EVT_FEAT_PERIODIC |
 				CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_DYNIRQ;
 	dw_ced->ced.set_state_shutdown = apbt_shutdown;