board: teensy: improve on-board SPI flash support

fixes: 29cc07ef - board: teensy: drop spi-nor from the flash nodes

The mentioned commit removed any sort of driver for the on board SPI flash
/ program region for the teensy4 boards. Due to this builds failed as the
available flash size / default program region size would automatically be
set to 0B.

With this patch the SPI driver has been updated to follow the
mimxrt1060_evk reference implementation.

Additional fixes include:
- changing size from bytes to bits for (teensy41)
- correcting of by one errors in size property
- explicitly stating write and erase block size
- moved to `DT_SIZE` and `DT_FREQ` for improved readability
- added `model` prop to `teensy41.dts`

Signed-off-by: Elisabeth Friedrich <auxsys@flowerpot.me>