mtd: rawnand: brcmnand: Allow SoC to provide I/O operations

Allow a brcmnand_soc instance to provide a custom set of I/O operations
which we will require when using this driver on a BCMA bus which is not
directly memory mapped I/O. Update the nand_{read,write}_reg accordingly
to use the SoC operations if provided.

To minimize the penalty on other SoCs which do support standard MMIO
accesses, we use a static key which is disabled by default and gets
enabled if a soc implementation does provide I/O operations.

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/20220107184614.2670254-3-f.fainelli@gmail.com