imx: lcdif: use one register structure for i.MXes

Share one lcdif structure for i.MXes.
1. Discard struct mxs_lcdif_regs from imx-regs.h of i.MX7
2. Add i.MX6SX/6UL/7D support in imx-lcdif.h of imx-common

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>