drivers: flash: flexspi_mx25um513: enforce write size limit in DTR mode

Per the MX25UM51325G datasheet, all page programs in OPI DTR mode need to
start at an even address, and be of even length. Update the minimum
write size reported by the driver and check all writes when OPI DTR mode
is enabled, so that subsystems using the flash driver can align to this
requirement.

Fixes #63639

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>