dts: arm: stm32h7 MPU attribute for the external Memory

Define the MPU attribute to be ATTR_MPU_EXTMEM for the
external region (qspi- or octo-spi NOR flash)
starting at 0x90000000 of the stm32h7 serie.
A XiP region should be Included inside with attribute
ATTR_MPU_IO, to access the external memory in XIP.
The stm32h7a/h7b serie as another external area at 0x70000000.

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