clocksource: arch_arm_timer: Fix age-old arch timer C3STOP detection issue

ARM arch timers are tightly coupled with the CPU logic and lose context
on platform implementing HW power management when cores are powered
down at run-time. Marking the arch timers as C3STOP regardless of power
management capabilities causes issues on platforms with no power management,
since in that case the arch timers cannot possibly enter states where the
timer loses context at runtime and therefore can always be used as a high
resolution clockevent device.

In order to fix the C3STOP issue in a way compliant with how real HW
works, this patch adds a boolean property to the arch timer bindings
to define if the arch timer is managed by an always-on power domain.

This power domain is present on all ARM platforms to date, and manages
HW that must not be turned off, whatever the state of other HW
components (eg power controller). On platforms with no power management
capabilities, it is the only power domain present, which encompasses
and manages power supply for all HW components in the system.

If the timer is powered by the always-on power domain, the always-on
property must be present in the bindings which means that the timer cannot
be shutdown at runtime, so it is not a C3STOP clockevent device.
If the timer binding does not contain the always-on property, the timer is
assumed to be power-gateable, hence it must be defined as a C3STOP
clockevent device.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Magnus Damm <damm@opensource.se>
Cc: Marc Carino <marc.ceeeee@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>