imx: imx-common: print i.MX 7 SoC names consistently

According to the product website, the full names are i.MX 7Solo
and i.MX 7Dual, whereas the short form is i.MX7S and i.MX7D. Be
consistent and print the short form for both supported i.MX 7 SoCs.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>