ARM: Kirkwood: make kirkwood_disable_mbus_error_propagation() static

The kirkwood_disable_mbus_error_propagation is not exported or declared
elsewhere, so make it static to avoid the following warning:

arch/arm/mach-mvebu/kirkwood.c:153:6: warning: symbol 'kirkwood_disable_mbus_error_propagation' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/mach-mvebu/kirkwood.c

@@ -150,7 +150,7 @@ static void __init kirkwood_dt_eth_fixup(void)
  * causes mbus errors (which can occur for example for PCI aborts) to
  * throw CPU aborts, which we're not set up to deal with.
  */
-void kirkwood_disable_mbus_error_propagation(void)
+static void kirkwood_disable_mbus_error_propagation(void)
 {
 	void __iomem *cpu_config;