mtd: rawnand: better name for the controller structure

In the raw NAND core, a NAND chip is described by a nand_chip structure,
while a NAND controller is described with a nand_hw_control structure
which is not very meaningful.

Rename this structure nand_controller.

As the structure gets renamed, it is logical to also rename the core
function initializing it from nand_hw_control_init() to
nand_controller_init().

Lastly, the 'hwcontrol' entry of the nand_chip structure is not
meaningful neither while it has the role of fallback when no controller
structure is provided by the driver (the controller driver is dumb and
can only control a single chip). Thus, it is renamed dummy_controller.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>