mtd: rawnand: stm32_fmc2: use regmap APIs

This patch uses regmap APIs to access all FMC2 registers.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1591975362-22009-6-git-send-email-christophe.kerello@st.com