sunxi: move #ifdef guards around tzpc_init() to header file

Some later 32-bit SoCs require some setup of the Secure Peripherals
Controller, which is handled in tzpc_init().
At the moment this is guarded in board.c by some #ifdefs selecting the
SoCs that need it.

Move those #ifdef guards into the header file, providing an empty stub
function for all other SoCs, so that the #ifdefs can be removed from the
.c file, to improve readability.

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