ARM: proc-v7: sanitise and document registers around errata

Document that r13 is not a stack in the initialisation function, in
case anyone gets other ideas.

Document the registers available for the errata workarounds, and
specifically which registers contain parts of the MIDR register, as
well as which registers must be preserved.

Lastly, use the lowest numbered available register (r0) rather than
r10 for temporary storage.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>