mtd: rawnand: stm32_fmc2: get resources from parent node

FMC2 EBI support has been added. Common resources (registers base
address and clock) can now be shared between the 2 drivers using
"st,stm32mp1-fmc2-nfc" compatible string. It means that the
common resources should now be found in the parent device when EBI
node is available.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1591975362-22009-7-git-send-email-christophe.kerello@st.com