rockchip: rk3188: use DM timer instead of rk_timer

Disable rk_timer as SYS timer and use DM timer instead,
so that we can get a better timer framework, the rk_timer
is going to be clean after we conver to use DM timer or
ARM arch/generic timer.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>