Original name, 'nvmemN', doesn't really give any hint about which
physical HW it may represent, so change the code to either pick up an
DT alias or use the name passed via 'struct nvmem_config'.

Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>