rtc: ds1511: put ds1511_nvmem_cfg on the stack

Move ds1511_nvmem_cfg to the stack of ds1511_rtc_probe. This results in a
very small code size reduction and make it safer on systems with two
similar RTCs:

   text	   data	    bss	    dec	    hex	filename
   2128	    164	      4	   2296	    8f8	drivers/rtc/rtc-ds1511.o.before
   2175	    100	      4	   2279	    8e7	drivers/rtc/rtc-ds1511.o.after

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