README.mx28_common: Rename it to README.mxs

commit 54965b61 (README: mxs: Introduce README.mxs) should have changed the
file name to README.mxs, as it covers both mx23 and mx28 now.

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