The PMC also needs to be online before the main cores. And many machines there's still an external EC on the board responsible for sequencing power state external to the chip. The x86 application cores actually start up quite late in the process, long after the SPI has been read out, memory and cache controllers initialized, etc...
x86 is similar, since Intel ME (part of the PCH, whether in the chip or not) is needed to boot the CPU.