ARM: stm32: Increase default SF bus frequency to 50 MHz and enable SFDP

Increase default SPI NOR bus frequency from 1 MHz to 50 MHz and
enable SFDP parsing to obtain more accurate SPI NOR configuration.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>