sunxi: sun50i_h6: make more clock functions SPL only

In clock_sun50i_h6.c, responsible for (mostly early) clock setup on
newer 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. By moving the clock_get_pll6()
function to the end of the file, all SPL-only clocks can be contained
within one #ifdef guard.

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>