boards: nxp: frdm_k22f: fix the flash size value

- The MK22FN512VLH12 chip, installed on frdm_k22f,
  has 512 KB of Program Flash and 128KB SRAM
  according to the K22P121M120SF7RM.pdf manual (page 55).
- Fix the flash size to 512KB (was 1MB).
- Add nxp_k22fn512.dtsi with correct flash size value.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>