sunxi: sun6i: make more clock functions SPL only

In clock_sun6i.c, responsible for (mostly early) clock setup on older
generation Allwinner SoCs, 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,
so 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. Include those functions into the existing CONFIG_SPL_BUILD guards,
so they are compiled for the SPL only.

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>