ARM: spear: build hotplug.o for armv7-a

The hotplug.c file uses assembly instructions that are only available
on ARMv7 but not on ARMv6. This is ok because we know that code will
only run on arm ARMv7 SPEARr13xx, but it produces build errors when
we also enable one of the ARMv6 targets in a multiplatform configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>