mtd: rawnand: stm32_fmc2: cosmetic change to use nfc instead of fmc2 where relevant

This patch renames functions and local variables.
This cleanup is done to get all functions starting by stm32_fmc2_nfc
in the FMC2 raw NAND driver when all functions will start by
stm32_fmc2_ebi in the FMC2 EBI driver.
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/1589284068-4079-2-git-send-email-christophe.kerello@st.com