ARM: 6288/1: ftrace: document mcount formats

Add a comment describing the mcount variants and how the callsites look
like.

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>