ARM: ux500: build hotplug.o for ARMv7-a

If we try to build this file in a multiplatform configuration with
ARMv6 enabled, gas complains about the dsb operation being undefined.
Adding -march=armv7-a is safe because that code is only ever run
on ux500, which is Cortex-A9.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>