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: Patrice Chotard <patrice.chotard@st.com>