nds32: To implement these icache invalidation APIs since nds32 cores don't snoop

nds32: To implement these icache invalidation APIs since nds32 cores don't snoop
data cache.
This issue is found by Guo Ren. Based on the Documentation/core-api/cachetlb.rst
and it says:

"Any necessary cache flushing or other coherency operations
that need to occur should happen here.  If the processor's
instruction cache does not snoop cpu stores, it is very
likely that you will need to flush the instruction cache
for copy_to_user_page()."

"If the icache does not snoop stores then this
routine(flush_icache_range) will need to flush it."

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>