ARM: outer cache: add documentation of outer cache functions

Add some documentation to cover the outer cache functions so that their
requirements can be better understood.  Of particular note are the
flush_all() and disable() methods which must not be called except in
very specific circumstances.

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

diff --cc arch/arm/include/asm/outercache.h
Simple merge