ARM i.MX25: Make timer irq work again

Since i.MX has SPARSE_IRQ enabled the i.MX25 timer is broken. This
is because the internal irqs now start at an offset of NR_IRQS_LEGACY.
The patch fixed this up, but missed the i.MX25 timer which used a
hardcoded value instead of a define. This patch introduces a define
for the timer irq and uses it.

This is broken since introduced with 3.6-rc1:

| commit 8842a9e2
| Author: Shawn Guo <shawn.guo@linaro.org>
| Date:   Thu Jun 14 11:16:14 2012 +0800
|
|    ARM: imx: enable SPARSE_IRQ for imx platform

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>