nvmem: dt: document SNVS LPGPR binding

Documentation bindings for the Low Power General Purpose Register
available on i.MX6 SoCs in the Secure Non-Volatile Storage.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>