boards: mimxrt1010_evk: set the flash device as enabled

Set the flash device node as "okay" so that it can be used with the
flash APIs and samples, also use the DT_SIZE_M macro for the flash node
itself so it's somewhat coherent with the second cell of the flexspi reg
property.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>