tee: add ARM_SMCCC dependency

For the moment, the tee subsystem only makes sense in combination with
the op-tee driver that depends on ARM_SMCCC, so let's hide the subsystem
from users that can't select that.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>