dt-bindings: mtd: update STM32 FMC2 NAND controller documentation

These bindings can be used on SOCs where the FMC2 NAND controller is
in standalone. In case that the FMC2 embeds 2 controllers (an external
bus controller and a raw NAND controller), the register base address,
the clock and the reset will be defined in the parent node.

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/1591975362-22009-3-git-send-email-christophe.kerello@st.com