clk: imx8mp: Add DISP2 pixel clock

Add pixel clock for second LCDIFv3 interface. Both LCDIFv3 interfaces use
the same set of parent clock, so deduplicate imx8mp_media_disp1_pix_sels
into common imx8mp_media_disp_pix_sels and use it for both.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Abel Vesa <abel.vesa@nxp.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20220313123949.207284-1-marex@denx.de
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>