ARM: dts: omap3-igep0020: use standard constant for IRQ flags

Commit 840ef8b7

 ("ARM: dt: add header to define IRQ flags") added
constants for IRQ edge/level triggered types so use it instead of
a magic number to enhance the DT readability.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>