dts: arm: stm32 reg definition for the st,stm32-ospi compatible

The st,stm32-ospi compatible is defining the reg property
with the register address and size at first index
followed by the external mem base address and max allocated size.
For the stm32H7, stm32L4plus, stm32L5, stm32U5 series.
ospi1 is addressing max 256 MBytes from 0x90000000
ospi2 is addressing max 256 MBytes from 0x70000000

Signed-off-by: Francois Ramu <francois.ramu@st.com>