soc: at91: Add Atmel SFR SN (Serial Number) support

Add support to read SFR's read-only registers providing the SoC
Serial Numbers (SN0+SN1) to userspace.

~ #  hexdump -n 8  -e'"%d\n"' /sys/bus/nvmem/devices/atmel-sfr0/nvmem
959527243
371539274

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20191004151802.21793-1-kamel.bouhara@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>