ARM: dts: keystone: Add missing unit name to interrupt controller

Add base address for GIC as unit address. This also squashes the
following warnings when built with W=1:
arch/arm/boot/dts/keystone-k2hk-evm.dtb: Warning (unit_address_vs_reg): Node /interrupt-controller has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2l-evm.dtb: Warning (unit_address_vs_reg): Node /interrupt-controller has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2e-evm.dtb: Warning (unit_address_vs_reg): Node /interrupt-controller has a reg or ranges property, but no unit name

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>