macintosh/rack-meter: Make of_device_ids const

macintosh/rack-meter: Make of_device_ids const

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    407	    576	      0	    983	    3d7	drivers/macintosh/rack-meter.o

File size after constify rackmeter_match.
   text	   data	    bss	    dec	    hex	filename
    807	    176	      0	    983	    3d7	drivers/macintosh/rack-meter.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>