drivers: flash_stm32_xspi: Fix Dummy Cycles on MX66UM1G45G NOR at 200MHz

This commit is fixing configuration of mx66uw1g45g NOR when working
at 200MHz.
According to its specification, when running at 200MHz, this memory should
use a Number Dummy Cycles configuration of 20 (DC bits in CFGR2), which is
the device's default configuration.
Applying the 66MHz configuration as done today was preventing flash to run
at frequency higher than 100Mhz.

This commit doesn't solve the more generic problem of this driver which
is applying this 66MHz configuration universally, irrespective of the
frequency and the memory device, but fixes the configuration which was
reported broken today.

Providing a global change would require starting a clear split between XSPI
controller configuration an bus device configuration, which is what new
MSPI API intend to solve, so this will be tackled once this driver will be
available.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>