jz4740: silence warnings related to mfd_get_cell changes

mfd_get_cell returns a const, so change the jz4740 clients to store
a const mfd cell.  This silences type mismatch warnings.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>