This revision adds the following parts:

• machine dependent low level init code
• SoC clocks detection and some utility functions
• Common interface to read/write/modify SoC system control registers, used by some of the other drivers and utility functions
• simple FDT resets support, based on the fdt_clock implementation already in the tree. For the moment resets and clocks are managed using these implementations. I am planning to port those to the new extres framework in the future, but currently I simply don't have time to do this part too.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5826