pico-imx7d: README: Recommend the usage of a USB hub

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: Fabio Estevam <festevam@gmail.com>