doc: boards: amlogic: update documentation for NanoPi-K2

Improve documentation. Notably we can now support U-Boot install to
the removable eMMC storage module in addition to SD cards.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/20230320114609.930145-15-christianshewitt@gmail.com
[narmstrong: fixed doc build]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
1 file changed