sunxi: compile clock.c for SPL only

With the clock_twi_onoff() function now being called only from the SPL,
the whole clock.c file in arch/arm/mach-sunxi is needed by SPL code
only.

Remove the redundant #ifdef from the clock_init() function, actually
this function was already only called from the SPL.
Then adjust the Makefile to compile clock.c only with CONFIG_SPL_BUILD
defined.

This avoids unnecessary code in U-Boot proper and allows further
refactoring and code-split between the SPL and U-Boot proper.

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