clocksource: convert 32-bit down counting clocksource on S5PV210/S5P64X0

Convert the S5PV210/S5P64X0 32-bit down-counting clocksource to
the generic mmio clocksource infrastructure

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>