mtd: spinand: add spinand_read_cfg() helper

Put REG_CFG reading code in spinand_read_cfg().
This function will be needed by the future SPI-NAND resume ops.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210602094913.26472-2-patrice.chotard@foss.st.com