board: arm: Add board support for mimxrt1020_evk

Add board support files for mimxrt1020_evk, the development board for
i.MXRT1021 (CM7) SoC.

- Add pinmux, dts, doc.
- Code can be loaded to SRAM.
- Tested samples: hello_world, philosophers, synchronization,
basic/blinky, and basic/button.

Signed-off-by: Ryan QIAN <jianghao.qian@nxp.com>