Use the new(-ish) CP15_SCTLR macro to generate system control reg accesses
where possible. In the places that doesn't work (multi-line inline asm,
and places where the old armv4 cpufuncs mechanism is used), annotate the
accesses with a comment that includes SCTLR. Now a grep -i sctlr can find
all the system control register manipulations.

No functional changes.