ARM: dts: DRA7: fix interrupt-cells for GPIO

GPIO modules are also interrupt sources. However, they require both the
GPIO number and IRQ type to function properly.

By declaring that GPIO uses interrupt-cells=<1>, we essentially do not
allow users of the nodes to use the interrupt property appropritely.

With this change, the following now works:

interrupt-parent = <&gpio6>;
interrupts = <5 IRQ_TYPE_LEVEL_LOW>;

Fixes: 6e58b8f1daaf ('ARM: dts: DRA7: Add the dts files for dra7 SoC and dra7-evm board')
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>