doc: fix references to README.qe_firmware

In two files README.qe_firmware is referenced which never made it into the
U-Boot tree. The README is available in the Linux kernel tree.

Update the references.

Cc: Timur Tabi <timur@kernel.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>