clk: sunxi-ng: h6: Set video PLLs limits

Video PLL factors can be set in a way that final PLL rate is outside
stable range. H6 user manual specifically says that N factor should not
be below 12. While it doesn't says anything about maximum stable rate, it
is clear that PLL doesn't work at 6.096 GHz (254 * 24 MHz).

Set minimum allowed PLL video rate to 288 MHz (12 * 24 MHz) and maximum
to 2.4 GHz, which is maximum in BSP driver.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>