nvmem: core: switch to device_property_present for reading property "read-only"

Switch to more generic device_property_present to consider also non-DT
properties.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>