dt-bindings: nvmem: Add i.MX IIM binding doc

The IIM is part of the i.MX device trees for long already, add a binding
document for it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>