Set the base address of translation table 0.

This fixes operation on Qualcomm Snapdragon and some other platforms.

During boot time on subsystems initialization we have some amount of
kernel threads created, then scheduler gives CPU time to each thread.
Eventually scheduler returns CPU execution back to thread 0. In this
case writing zero to ttbr0 in cpu_switch leads Qualcomm board to
reboot (asynchronously, CPU continues execution).

Similar to other kernel threads install a valid physical address
(kernel pmap) to user page table base register ttbr0.

Reviewed by: andrew
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D13536