csky: BUGFIX pfn_valid() for highmem

The variable "high_memory" is higmem_start. If the pfn is in highmem
then the return value of pfn_valid() will be false, but we expect true.

This will cause problem in __update_cache():

	pfn = pte_pfn(pte);
	if (unlikely(!pfn_valid(pfn)))
		return;

It will directly return without flush_cache.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>