drm: mxsfb: Add i.MX7 and i.MX8M to the list of supported SoCs in Kconfig

Extend the Kconfig option description by listing the i.MX7 and i.MX8M
SoCs, as they are supported by the same driver. Replace the list of SoCs
in the short description with just "(e)LCDIF LCD controller" to avoid
expanding it further in the future as support for more SoCs is added.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-17-laurent.pinchart@ideasonboard.com