Since commit 9e3c0174 ("pico-imx7d: Add LCD support") we started to
notice some hangs in U-Boot.

There is not an issue on such commit per se, but due to the LCD support
the current drawn is increased and this may cause issues when powering
pico-imx7d-pi from USB.

Some computers may be a bit strict with USB current draw and will
shut down their ports if the draw is too high.

The solution for that is to use an externally powered USB hub between the
board and the host computer.

Add such recommendation to the README file.
Signed-off-by: NFabio Estevam <festevam@gmail.com>

Update the README file to take into accound the switch to SPL.
Signed-off-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NFabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

Our default config already has the secure mode supported, so the
manual step is not required anymore.
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

The DFU allows a more user friendly use as the details where the
bootloader is installed are abstracted.
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

The NXP 4.1 kernel needs to boot with secure boot.

Add information on how to enable secure boot mode.
Signed-off-by: NVanessa Maegima <vanessa.maegima@nxp.com>

Add the initial support for pico-imx7d board based on Wig Cheng's
source code.

Add support for eMMC, USB gadget, I2C, PMIC and Ethernet.

For more information about this board, please visit:
http://www.technexion.org/products/pico/pico-som/pico-imx7-emmcSigned-off-by: NVanessa Maegima <vanessa.maegima@nxp.com>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>