ARM: dts: rockchip: temporarily disable smp on rk3288

Stock firmware on rk3288 does not initizalize the CNTVOFF registers
of the architected timer correctly. This introduces issues with the
newly added SMP support for rk3288, resulting in rcu stalls due to
differing timer values per core.

There exist preliminary and tested patches for u-boot for this problem,
but there are a minority of boards using other bootloaders like coreboot.

There also is currently a second solution for miss-initialized architected
timers in the works:
- clocksource: arch_timer: Fix code to use physical timers when requested
- clocksource: arch_timer: Allow the device tree to specify uninitialized timer registers

Therefore disable smp on rk3288 again till these are finalized, also
allowing coreboot-based boards to boot again.

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