imx: Add SeeedStudio NPI-IMX6ULL Support

CPU: Freescale i.MX6ULL rev1.1 792 MHz (running at 396 MHz)
CPU: Industrial temperature grade (-40C to 105C) at 49C
Reset cause: POR
Model: Seeed NPi iMX6ULL Dev Board with NAND
Board: Seeed NPi i.MX6ULL Dev Board
DRAM: 512 MiB
NAND: 512 MiB
MMC: FSL_SDHC: 0
In: serial@2020000
Out: serial@2020000
Err: serial@2020000
Net: FEC0

Working:
- Eth0
- MMC/SD
- NAND
- UART 1
- USB host

Signed-off-by: Navin Sankar Velliangiri <navin@linumiz.com>

Note:

Changes in v2:

* removed unnecessary space in imx6ull-seeed-npi-imx6ull-dev-board.dts file.
* Used SZ_2M for CONFIG_SYS_MALLOC_LEN size allocation.