mfd: syscon: Add Atmel GPBR DT bindings documention

The GPBR block provides a set of battery-backed registers that can be used
to save data which need to be kept when the system is powered down and
VDD-core is maintained by an external battery.

A typical usage is the RTT block (when used as an RTC) which needs one of
those registers to save the current time.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>