mtd: rawnand: brcmnand: Move Kconfig to driver folder

In preparation for allowing each of the brcmnand stub to be built
separately, move the Kconfig entry to the driver folder.

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/20220711222323.4048197-2-f.fainelli@gmail.com