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>