ARM: orion5x: make orion5x_legacy_handle_irq static

The orion5x_legacy_handle_irq() is not used or declared outside
of irq.c so make it static to avoid the following warning:

arch/arm/mach-orion5x/irq.c:30:23: warning: symbol 'orion5x_legacy_handle_irq' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>