drm/sun4i: mixer: Extend regmap max_register

Better guess. Secondary CSC registers are from 0xF0000.

Signed-off-by: Martin Cerveny <m.cerveny@computer.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200906162140.5584-3-m.cerveny@computer.org