SPL: Port SPL framework to powerpc

This patch enables the SPL framework to be used on powerpc platforms
and not only ARM.

timer_init() does not exist on PPC systems. The timer (decrementer) is
initialized and enabled in interrupt_init() here. And currently
interrupt_init() is called after relocation to SDRAM. Since the only
powerpc SPL implementation (a3m071) doesn't need a timer, let's remove
this timer_init() call for PPC systems.

Signed-off-by: Stefan Roese <sr@denx.de>