irqchip: omap-intc: enable IP protection

When PROTECTION bit in enabled in PROTECTION
register, INTC's registers are only accessible
from privileged mode.

Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>