ARM: dts: stm32: Fix qspi memory map size for stm32f7 boards

stm32f746-disco embeds a 16Mb qspi flash, stm32f746-eval and
stm32f769-disco embeds a 64Mb qspi flash.
Update the reg property accordingly

Remove deprecated memory-map property.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>