boards: arm: stm32 disco kit with octoflash description

No sfdp-table property given by the DTS but received from
the octoflash Node rely on the issued by the read sfdp command.
Note that the size of the mx25lm51245 flash controller
is expressed in bits (ie 512Mbits or 64 Mbytes).

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