rockchip: config: tiner-rk3288: extend CONFIG_SYS_MONITOR_LEN to 600KB

The raw u-boot.bin for tinker board has been about 450KB without
debug option, and 550KB with all debug on, and the default value is 200KB,
which is not enough for run raw u-boot.bin.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>