boards: arm: stm32h7b3 disco kit has external octo NOR flash controller

Add the alias to the MX25LM512 octo-NOR flash for the spi-flash node.
Remove the overlay for the samples/drivers/spi_flash as no MDMA driver
exists yet for the stm32h7.

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