ARM: spear: use common irqchip_init function

Convert spear DT irq initialization over to use common irqchip_init
function.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Viresh Kumar <viresh.linux@gmail.com>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: Rajeev Kumar <rajeev-dlh.kumar@st.com>