ARM: rockchip: enable arm-global-timer

RK2928 and RK3066 contain a dw_apb timer component, while the rk3188 uses
a slightly similar but still different timer component. But all of them
support the ARM-global-timer that got added as clocksource driver recently.

So enable support for it to get a working clocksource for rk3188.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>