ARM: 8484/1: Documentation: l2c2x0: Mention separate controllers explicitly

The documentation in l2c2x0.txt is only valid for L2C210/L2C220/L2C310
(also known as PL210/PL220/PL310 and variants). Mention this explicitly.
And add a note why this isn't valid for integrated L2 controllers.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>