csky/mm: Fixup export invalid_pte_table symbol

There is no present bit in csky pmd hardware, so we need to prepare invalid_pte_table
for empty pmd entry and the functions (pmd_none & pmd_present) in pgtable.h need
invalid_pte_talbe to get result. If a module use these functions, we need export the
symbol for it.
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>
Cc: Mo Qihui <qihui.mo@verisilicon.com>
Cc: Zhange Jian <zhang_jian5@dahuatech.com>

arch/csky/mm/init.c

@@ -31,6 +31,7 @@
 
 pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
 pte_t invalid_pte_table[PTRS_PER_PTE] __page_aligned_bss;
+EXPORT_SYMBOL(invalid_pte_table);
 unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
 						__page_aligned_bss;
 EXPORT_SYMBOL(empty_zero_page);