clocksource/drivers/arc_timer: Update some comments

TIMER0 interrupt ACK is different for ARC700 and HS3x cores.

This came to light in some internal discussions and it is nice to have this
documented rather than digging up the PRM (Programmers Reference Manual).

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Link: https://lkml.kernel.org/r/1519241491-12570-1-git-send-email-vgupta@synopsys.com