mtd: rawnand: brcmnand: Move OF operations out of brcmnand_init_cs()

In order to initialize a given chip select object for use by the
brcmnand driver, move all of the Device Tree specific routines outside
of brcmnand_init_cs() in order to make it usable in a platform data
configuration which will be necessary for supporting BCMA chips.

No functional changes introduced.

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/20220107184614.2670254-5-f.fainelli@gmail.com