to play with uboot, quick and dirty way is:
mw.l 0xff000034 0x0ff00440
revisited ii:
with this we can go better 
revisited iii:
seems all cortex-a35 chips from rockchip (rk1808, px30, rk3308, rk3326) have jtag pins enabled, rk3308.cfg merged to master of openocd, will be in 0.11.0