Introduce boolean property st,non-secure-otp for OTP data located
in a factory programmed area that only secure firmware can access
by default and that shall be reachable from the non-secure world.

This change also allows additional properties for NVMEM nodes that
were forbidden prior this change.
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>