nvmem: add snvs_lpgpr driver

This is a driver for Low Power General Purpose Register (LPGPR)
available on i.MX6 SoCs in Secure Non-Volatile Storage (SNVS)
of this chip.

It is a 32-bit read/write register located in the low power domain.
Since LPGPR is located in the battery-backed power domain, LPGPR can
be used by any application for retaining data during an SoC power-down
mode.

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