sunxi: sun50i_h6: add A523 SPL clock setup code

sunxi: sun50i_h6: add A523 SPL clock setup code

This adds the early A523 clock setup code, for the basic peripheral PLL
and the basic bus clocks (APB/AHB). This is quite close to the existing
H6 and H616 clock code, so this shares the same file. A few bits and bobs
are different, though, so filter for the A523 in a few occasions.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>