ARM: dts: BCM5301X: Add interrupt properties to GPIO node

The GPIO controller is also an interrupt controller provider and is
currently missing the appropriate 'interrupt-controller' and
'#interrupt-cells' properties to denote that.

Fixes: fb026d3d

 ("ARM: BCM5301X: Add Broadcom's bus-axi to the DTS file")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>