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>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>