ARM: kirkwood: fix typo in Makefile.boot

Just a small typo fix to make lsxl dtbs compile

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@googlemail.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>