ARM: OMAP3: Remove legacy IRQ for PRM

We have this coming from device tree and legacy booting is
no longer needed.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>