mfd: Don't declare jz4740_adc_cells const

Remove the const keyword to fix below warning:

  CC      drivers/mfd/jz4740-adc.o
drivers/mfd/jz4740-adc.c: In function 'jz4740_adc_probe':
drivers/mfd/jz4740-adc.c:290: warning: passing argument 3 of 'mfd_add_devices' discards qualifiers from pointer target type
include/linux/mfd/core.h:93: note: expected 'struct mfd_cell *' but argument is of type 'const struct mfd_cell *'

Also make jz4740_adc_cells static, is not used outside
this driver so no need to make the symbol global.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>