mfd: Inherit coherent_dma_mask from parent device

dma_mask and dma_parms are already inherited from the parent device but
dma_coherent_mask was left uninitialized (set to zero thanks to kzalloc).
Set sub-device coherent_dma_mask to its parent value to simplify
sub-drivers making use of dma coherent helper functions (those drivers
currently have to explicitly set the dma coherent mask using
dma_set_coherent_mask function).

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>