verdin-imx8mm: add initial setup

Add initial verdin.mk for the minimal setup for OP-TEE testing.
This setup uses latest mainline TF-A (v.2.2) / U-Boot (v2020.03) /
Linux (5.6-rc).

Verdin i.MX8M Mini boot sequence:

BootROM -> SPL -> TF-A (BL31) -> OP-TEE (BL32)
                   |
                   -> U-Boot (BL33) -> Linux

TF-A(bl31.bin), U-Boot proper and OP-TEE (tee.bin) are packed in FIT image,
which is parsed by SPL.

Deploying:

1. Flasing flash.bin (IMX ready-to-boot image):
> dhcp && tftpboot ${loadaddr} flash.bin
> setexpr blkcnt ${filesize} + 0x1ff
> setexpr blkcnt ${blkcnt} / 0x200
> mmc dev 0 1 && mmc write ${loadaddr} 0x2 ${blkcnt}

2. Boot Linux kernel via TFTP/NFS:

-> setenv serverip <ip_of_your_server>
-> setenv nfsroot /path/to/nfs/root
-> setenv fdt_file fsl-imx8mm-verdin-dev.dtb
-> setenv bootargs "console=${console},${baudrate} root=/dev/nfs \
            ip=dhcp nfsroot=${serverip}:${nfsroot},v3,tcp"
-> tftp ${loadaddr} ${image}; tftp ${fdt_addr} ${fdt_file};
-> booti ${loadaddr} - ${fdt_addr}

Test results:

+-----------------------------------------------------
24600 subtests of which 0 failed
98 test cases of which 0 failed
0 test cases were skipped
TEE test application done!

Acked-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>