clocksource: arm_arch_timer: add structs to describe MMIO timer

In preparation for ACPI GTDT support, this patch adds structs to
describe the MMIO timers indepedent of the firmware interface.

Subsequent patches will use these to split the FW/HW probing logic, so
that the HW probing logic can be shared by ACPI and DT.

Signed-off-by: Fu Wei <fu.wei@linaro.org>
Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>