dts: arm: st: Add Flash Controller Nodes for STM32{F0,F3,F4,L4}

Introduce flash controller nodes for STM32 SoCs that are supported in
the flash driver.  This is a precusor to converting the flash driver on
stm32 over to using device tree.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>