drivers: flash: flash_stm32_qspi: Add support for cs-high-time

This commit adds support for the new cs-high-time devicetree property.
The QUADSPI_DCR_CSHT is now configured according to the value indicated
in the devicetree, for both single and dual flash modes.

Signed-off-by: Thomas Altenbach <altenbach.thomas@gmail.com>