[MIPS] Export local_flush_data_cache_page for sake of IDE.

On a CPU with aliases the IDE core needs to flush caches in the special
IDE variants of insw, insl etc.  If IDE support is built as a module this
will only work if local_flush_data_cache_page happens is exported as a
module.

As per policy export local_flush_data_cache_page as GPL symbol only.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>