optee: add writeback to valid memory type

Only in smp systems the cache policy is setup as write alloc, in
single cpu systems the cache policy is set as writeback and it is
normal memory, so, it should pass the is_normal_memory check in the
share memory registration.

Add the right condition to make it work in no smp systems.

Fixes: cdbcf83d ("tee: optee: check type of registered shared memory")
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>