rtc: m48t86: put m48t86_nvmem_cfg on the stack

Move m48t86_nvmem_cfg to the stack of m48t86_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
   1733	    164	      0	   1897	    769	drivers/rtc/rtc-m48t86.o.before
   1793	    100	      0	   1893	    765	drivers/rtc/rtc-m48t86.o.after

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