[zircon][msm8x53] Initial bringup on msm8x53 SOM

Adapted from work by voydanoff.

Still some work to do:
-Needed to relocate kernel to 0xa000.0000+ to work around what appears
to be some sort of memory-stomping situation.
-Remove hack to touch bad physical memory to reboot the device once we
find a proper way to reset the soc.

NOTES:

Device is flashed via fastboot using scripts/flash-msm8x53-som

Power cycling with volume down button pressed gets you into fastboot.
The system will then reboot and start zircon. Holding volume down on
subsequent reboot will go back into fastboot.

Tested: boot on msm8x53 som and validate that qemu-arm and vim2 still boot.

Change-Id: I44c594d75644b763c00f4373a63216e2f6b1fff7