vexpress-poweroff: Should depend on the required infrastructure

ARM Versatile Express reset driver requires platform-specific config
infrastructure to be present in the kernel. When VEXPRESS_CONFIG is not
selected, the build will fail like this:

drivers/built-in.o: In function `vexpress_reset_do.clone.0':
iio-trig-interrupt.c:(.text+0x1aff38): undefined reference to `__vexpress_config_func_get'
iio-trig-interrupt.c:(.text+0x1aff4c): undefined reference to `vexpress_config_write'

Added required dependency to the Kconfig entry.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>