ARM: ixp4xx: Add myself as maintainer

I am working on the platform right now so might as well
maintain it for a bit.
Suggested-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -1685,6 +1685,7 @@ L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
 
 ARM/INTEL IXP4XX ARM ARCHITECTURE
+M:	Linus Walleij <linusw@kernel.org>
 M:	Imre Kaloz <kaloz@openwrt.org>
 M:	Krzysztof Halasa <khalasa@piap.pl>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)