boards: arm: Enable flash for storage on mimxrt595_evk

- The MX25UM51345G flash is connected to FLEXSPI PortA for
  mimxrt595_evk.
- Updated flexspi_mx25um51345g driver to support DTR OPI mode.
- Tested with tests/drivers/flash.

Signed-off-by: Chay Guo <changyi.guo@nxp.com>
12 files changed