MIPS: Loongson64: Mark RS780 HPET as broken

This driver is using some dangerous hack to set MMIO address for HPET,
which might break systems with other kinds of PCH.

Also, as Loongson-3 cpufreq driver never appeared in mainline,
this driver rarely got used.

So we temporarily mark it as broken until we find a better solution.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>