mtd: rawnand: brcmnand: Allow platform data instantation

Make use of the recently refactored code in brcmnand_init_cs() and
derive the chip-select from the platform data that is supplied. Update
the various code paths to avoid relying on possibly non-existent
resources, too.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220107184614.2670254-8-f.fainelli@gmail.com