clk: sunxi-ng: add missing hdmi-slow clock for H6 CCU

The Allwinner H6 CCU has a "HDMI Slow Clock", which is currently missing
in the ccu-sun50i-h6 driver.

Add this missing clock to the driver.

Fixes: 542353ea ("clk: sunxi-ng: add support for the Allwinner H6 CCU")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>