tee: optee: making OPTEE_SHM_NUM_PRIV_PAGES configurable via Kconfig

This change adds KCONFIG option to set number of pages out of
whole shared memory to be used for OP-TEE driver private data
structures.

Signed-off-by: Sahil Malhotra <sahil.malhotra@nxp.com>
[jw: fixing trivial merge conflict]
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>