In order to get the APQ8016 to even boot you need the PM8916, the ROM boot code in the processor sends commands to the PMIC to power up the supplies to the right levels in the right sequence before the first line of code is fetched from the eMMC. The Boot sequence even sends commands to the PM8916 to power up the eMMC (and the SDCard in the case of SD boot). The PM8916 provides the many voltages required for the APQ8016 to operate (including the DRAM), and ensures the correct power sequencing. Additionally the PM8916 provides the Fast Transient Switchers that are absolutely required to allow the A53 cores to operate. Take a look at the Processor Design Guidelines to gain a understanding of the complexity of the power systems for this SOC. Any implementation that you could possibly create with discrete switchers and a micro controller would be much larger and more expensive than the fully custom solution provided by the PM8916.
The PM8916 also contains the Audio ADC and DACs, if you want to use some other audio solution then you can connect your audio solution to the I2S pins on the SOC and simply not use the audio solution build into the PM8916.
Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.
