mtd: spinand: Add spinand_init_flash() helper

Add spinand_init_flash() helper which implement
all needed init for 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/20210602094913.26472-3-patrice.chotard@foss.st.com