sun6i: display: Add support for using the mipi pll as lcd clock source

Add support for using the mipi pll as lcd clock source, this is
necessary for getting higher dotclocks with lcd panels.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Anatolij Gustschin <agust@denx.de>