rockchip: rk3188: add timer3 node

Add dts node for timer3.
Because of the rockchip timer can only KNOWN "dtd_rockchip_rk3368_timer"
with OF_PLATDATA enable, so we override its compatible to
"rockchip,rk3368-timer".

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