clocksource: arm_arch_timer: Change clocksource name if CP15 unavailable

The arm and arm64 VDSOs need CP15 access to the architected counter.
If this is unavailable (which is allowed by ARM v7), indicate this by
changing the clocksource name to "arch_mem_counter" before registering
the clocksource.

Suggested by Stephen Boyd.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>