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>