UPSTREAM: nvmem: Add flag to export NVMEM to root only

UPSTREAM: nvmem: Add flag to export NVMEM to root only

Legacy AT24, AT25 EEPROMs are exported in sys so that only root can
read the contents. The EEPROMs may contain sensitive information. Add
a flag so the provide can indicate that NVMEM should also restrict
access to root only.

Change-Id: Ida22fb0ea22aef379e1bd0be241725eb96c31789
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>
(cherry pick from 811b0d6538b9f26f3eb0f90fe4e6118f2480ec6f)