board/freescale/common/imx: add support for i.MX8M Nano

board/freescale/common/imx: add support for i.MX8M Nano

Almost identical as i.MX8M Mini, here are the differences:
- different ATF load address [1]
- different entry point [2]
- different ROM version (v2) [3]

[1] https://gitlab.denx.de/u-boot/u-boot/-/blob/v2020.04/board/freescale/imx8mn_evk/README
[2] https://gitlab.denx.de/u-boot/u-boot/-/blob/v2020.04/configs/imx8mn_ddr4_evk_defconfig#L22
[3] https://gitlab.denx.de/u-boot/u-boot/-/commit/b8f16834

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>