board/freescale/common/imx: add support for i.MX 8M Plus

board/freescale/common/imx: add support for i.MX 8M Plus

Almost identical as i.MX 8M Nano, with a couple of differences:
- different ATF load address [1]
- different entry point [2]

[1] https://source.denx.de/u-boot/u-boot/-/commit/e8e2703a3050feb8d2e6473d806c5277d5e3236f
[2] https://source.denx.de/u-boot/u-boot/-/blob/v2021.04-rc4/configs/imx8mp_evk_defconfig#L14

Signed-off-by: Stephane Viau <stephane.viau@oss.nxp.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>