memory: stm32-fmc2-ebi: add STM32 FMC2 EBI controller driver

The driver adds the support for the STMicroelectronics FMC2 EBI controller
found on STM32MP SOCs.

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/1591975362-22009-5-git-send-email-christophe.kerello@st.com