video: Add information about using TrueType fonts

U-Boot supports using TrueType fonts on the console, which is useful for
presenting a nice UI to users, e.g. for system recovery.

Add information about how to compile this on ARM platforms, since this is
not obvious.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>