mtd: rawnand: MTD_NAND_BCM47XXNFLASH needs CONFIG_BCMA

We already have a dependency on BCMA_NFLASH, which in turn depends on
BCMA, but since BCMA is a tristate option and BCMA_NFLASH is bool,
we can run into an invalid configuration with MTD_NAND_BCM47XXNFLASH=y
and BCMA=m:

drivers/mtd/nand/raw/bcm47xxnflash/ops_bcm4706.o: In function `bcm47xxnflash_ops_bcm4706_init':
ops_bcm4706.c:(.text+0x790): undefined reference to `bcma_chipco_pll_read'

Adding the dependency here forces MTD_NAND_BCM47XXNFLASH to only be
configured =m here so it can link against the BCMA driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>