doc: README.mx28_common: Add missing entry into Contents

commit 419ea2d84 (mx28: Split the README into a common part and a m28 specific part)
missed an entry in the Contents section that was part of the original file.

Add the missing entry for completeness.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
1 file changed