Bluetooth: Remove conn & SMP dependency from BLUETOOTH_TINYCRYPT_ECC

Ever since we expose ECDH as a more generic API that's not bound to
SMP we shouldn't be requiring selcting SMP in the configuration for it
to be available. Move it outside of the BLUETOOTH_SMP and
BLUETOOTH_CONN dependencies.

Change-Id: I4ac15ff92b1d4fcf3061a487718d364aa7d43bc8
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>