drm/imx: ipuv3-crtc: Allow to divide DI clock from TVEv2

This patch allows the IPU to divide the 27 MHz input clock from
the TVE by two to obtain the 13.5 MHz pixel clock needed for
NTSC/PAL SD modes.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>