sunxi: sun9i: make more clock functions SPL only

In clock_sun9i.c, responsible for (mostly early) clock setup on the
Allwinner A80 SoC, many functions are only needed by the SPL, and are
thus already guarded by CONFIG_SPL_BUILD.

Over the years drivers like for the UART or I2C were converted to DM, and
they care about clock setup themselves now, by using a proper DM clock
driver.

This means those devices need the clock setup functions here for the SPL
only. Move some functions around, to group all SPL-only function within
one #ifdef guard. Some functions were exported, but never used outside
of this file, so remove their prototypes from the header file and mark
them as static.

This avoids unnecessary code in U-Boot proper and helps further
refactoring. Add some comments on the way to help understanding of the
file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>