ion: fix unmapped heap test settings **not for mainline**

ion: fix unmapped heap test settings **not for mainline**

If one enables ION_DUMMY_UNMAPPED_HEAP without providing the target
unmapped heap configuration settings (physical base address and size),
the kernel cannot build. This situation occurs in Linux test build
cases, i.e running the allmodconfig configuration.

This change overcomes the issue by providing default null settings for
both ION_DUMMY_UNMAPPED_BASE and ION_DUMMY_UNMAPPED_SIZE.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit ac0c2c26b9819c5e95d56cb2d8937de0357eecaa
 linaro repo https://github.com/linaro-swg/linux.git
 tag optee-v4.9-20171005)