ARM: hisi: depend on ARCH_MULTI_V7

hisi has a general dependency on ARCH_MULTIPLATFORM, which is
problematic when building a kernel for non-V7 platforms but selecting
drivers that might conflict with other architecture levels.

In this case, it broke my (still out of tree) patch set that
enables V7M multiplatform support, since that does not enable
MULTI_IRQ support:

arch/arm/kernel/built-in.o: In function `set_handle_irq':
arch/arm/kernel/irq.c:125: undefined reference to `handle_arch_irq'
arch/arm/kernel/built-in.o: In function `setup_arch':
arch/arm/kernel/setup.c:965: undefined reference to `handle_arch_irq'

Since all hisilicon platforms are ARMv7 based, we can avoid this
problem by just making the dependency more specific.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Wei Xu <xuwei5@hisilicon.com>