arm: dts: rockchip: rk3188-radxarock-u-boot: remove timer compatible replacement

The Rockchip timer driver has been renamed after the fall back compatible.
There's no need to replace the timer compatible in rk3188-radxarock-u-boot.dtsi
anymore, so remove.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>