boards: arm: add board support for mimxrt1015_evk

Add board support files for mimxrt1015_evk, the development board for
i.mxrt1015 (CM7) SoC.

- Add pinmux, dts, and doc.
- Tested samples: hello_world, philosophers, synchronization,
basic/blinky, basic/button.

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