ARM: 8458/1: bL_switcher: add GIC dependency

It is not possible to build the bL_switcher code if the GIC
driver is disabled, because it relies on calling into some
gic specific interfaces, and that would result in this build
error:

arch/arm/common/built-in.o: In function `bL_switch_to':
:(.text+0x1230): undefined reference to `gic_get_sgir_physaddr'
:(.text+0x1244): undefined reference to `gic_send_sgi'
:(.text+0x1268): undefined reference to `gic_migrate_target'
arch/arm/common/built-in.o: In function `bL_switcher_enable.part.4':
:(.text.unlikely+0x2f8): undefined reference to `gic_get_cpu_id'

This adds a Kconfig dependency to ensure we only build the big-little
switcher if the GIC driver is present as well.

Almost all ARMv7 platforms come with a GIC anyway, but it is possible
to build a kernel that disables all platforms.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>