ANDROID: ion: export some ion symbols

Background:
  We can allocate buffer from ion_system_heap for general case,
if the buffer is needed to access by HW, we need to map/unmap
dma_address by iommu driver before and after HW engine access for
iommu case. For example, if the same buffer is accessed by three
HW engines, then every device driver needs to map/unmap dma_address
by iommu driver respectively.
  For MTK platform, all the multimedia HW engines use the same
iommu HW and share the same iommu page table, so multiple map/unmap
dma_address by iommu driver maybe reduce performance seriously for
above case, especially for big size buffer. In order to improve
performance for MTK platform, we want to add ion_mtk_iommu_heap
and implement map/unmap dma_address by ion_mtk_iommu_heap:
  a.heap->buf_ops.map_dma_buf: If more than one attachments map
dma_address by iommu driver about the same buffer, the function
only map once in first attachment, the other attachments of buffer
copy sg_table which has already mapped to respective sg_table directly
and don't mapping dma_address truly.
  b.heap->buf_ops.unmap_dma_buf: Do nothing.
  c.heap->buf_ops.release: unmap dma_address by iommu driver about buffer
and free the buffer.

Based on above description:
We will build ion_mtk_iommu_heap as a kernel module and ion_mtk_iommu_heap
also use some APIs like ion_system_heap but they not do export symbol
ion_page_pool_alloc()
ion_page_pool_free()
ion_page_pool_nr_pages()
ion_page_pool_shrink()
ion_page_pool_create()
ion_page_pool_destroy()

Bug: 158729041
Change-Id: If3897d5e0105edefe03a51ac52c7b3d423aaa988
Signed-off-by: Chao Hao <chao.hao@mediatek.com>
Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>