Revert "mfd: core: Un-constify mfd_cell.of_reg"

This reverts commit 94eb5423 which is
commit 3c70342f upstream.

It breaks the Android ABI and if it is really needed can come back in
the future in an ABI-safe way.

Bug: 161946584
Change-Id: I31531da478d4a3e87c532ab9c85980415e07b945
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>