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.