staging: android ion/hisi: fix dependencies

The newly added Hi6220 Ion code fails to build when the ION_OF helpers
are not present:

drivers/staging/android/ion/hisilicon/hi6220_ion.o: In function `hi6220_ion_remove':
hi6220_ion.c:(.text.hi6220_ion_remove+0x4c): undefined reference to `ion_destroy_platform_data'
drivers/staging/android/ion/hisilicon/hi6220_ion.o: In function `hi6220_ion_probe':
hi6220_ion.c:(.text.hi6220_ion_probe+0x5c): undefined reference to `ion_parse_dt'
hi6220_ion.c:(.text.hi6220_ion_probe+0xf8): undefined reference to `ion_destroy_platform_data'

This selects the symbol when needed.

Fixes: 2b40182a ("staging: android: ion: Add ion driver for Hi6220 SoC platform")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>