[mips/ar531x] code cleanup, non-INTRNG support

This commit improves code styles like:

• removing commented code
• format comments as C-style
• add spaces after #define-s

It also bring ability to build kernel without INTRNG and remove RedBoot dependency.

Tested on FON2201

Submitted by: Hiroki Sato <yamori813@yahoo.co.jp>
Reviewed by: adrian, mizhka
Approved by: adrian(mentor)
Differential Revision: https://reviews.freebsd.org/D8557