sched_clock: document 4Mhz vs 1Mhz decision

sched_clock: document 4Mhz vs 1Mhz decision

Bo Shen sent a patch to change this to 1Mhz instead of 4Mhz but according
to Russell King the use of 4Mhz was intentional.  Add a comment to this
effect so that others don't try to change the code as well.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Bo Shen <voice.shen@atmel.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>