sunxi: psci: clean away preprocessor macros

sunxi: psci: clean away preprocessor macros

This patch restructures psci.c to get away from the "many different
function definitions switched by #ifdef" paradigm to the preferred style
of having a single function definition with `if (IS_ENABLED(...))` to
make the optimizer include only the appropriate function bodies instead.

There are no functional changes here.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>