clocksource/drivers/arm_arch_timer: Add standalone MMIO driver

clocksource/drivers/arm_arch_timer: Add standalone MMIO driver

Add a new driver for the MMIO side of the ARM architected timer.
Most of it has been lifted from the existing arch timer code,
massaged, and finally rewritten.

It supports both DT and ACPI as firmware descriptions.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20250814154622.10193-3-maz@kernel.org