linux-imx

doc: replace FTP URL to kernel.org with HTTPS one

FTP services were shutdown some weeks ago, so the FTP URL
does not work anymore. Fix this by replacing it with
corresponding HTTPS URL.

Signed-off-by: Michael Heimpold <michael.heimpold@i2se.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>