ARM: Improve robustness of locore_v6.S and fix errors.
- boot page table is not allocated in data section, so must be cleared before use
- map only one section (1 MB) for SOCDEV mapping (*)
- DSB must be used for ensuring of finishing TLB operations
- Invalidate BTB when appropriate
PR: 198360
Reported by: Daisuke Aoyama <aoyama at peach.ne.jp> (*)
Approved by: kib (mentor)