ARM: spear: convert to MULTI_IRQ_HANDLER

Now that there is a generic IRQ handler for multiple VIC devices use it
for spear to help building multi platform kernels.

Acked-by: Viresh Kumar <viresh.kumar@st.com>
Cc: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>