sunxi: spl: Fix H616 clock initialization

It turns out that there is a magic bit in PRCM region which seemingly
makes PLLs work if it's enabled. Sadly, there is no documentation what
it does exactly, so we'll just mimick BSP boot0 behaviour and enable it
before any clock is set up.

Fixes: b18bd53d ("sunxi: introduce support for H616 clocks")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>