media: ddbridge/mci: make ddb_mci_cmd() and ddb_mci_config() public

In preparation for splitting all MaxSX8 related code parts from the common
MCI code, prefix both mci_cmd() and mci_config() functions with ddb_,
remove the static marking and add matching function prototypes to
ddbridge-mci.h so these functions can be reused from other files within
the ddbridge driver. As this requires the mci-related structs to be
defined in ddbridge-mci.h, move struct mci and struct mci_base there and
clean them up.

Picked up from the upstream dddvb GIT.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>