ARM: davinci: dm646x: fix timer interrupt generation

commit b3843414 ("ARM: davinci: irqs: Correct McASP1 TX interrupt
definition for DM646x") inadvertently removed priority setting for
timer0_12 (bottom half of timer0). This timer is used as clockevent.

When INTPRIn register setting for an interrupt is left at 0, it is
mapped to FIQ by the AINTC causing the timer interrupt to not get
generated.

Fix it by including an entry for timer0_12 in interrupt priority map
array. While at it, move the clockevent comment to the right place.

Fixes: b3843414 ("ARM: davinci: irqs: Correct McASP1 TX interrupt definition for DM646x")
Signed-off-by: Sekhar Nori <nsekhar@ti.com>