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>

drivers/power/reset/Kconfig

@@ -40,7 +40,7 @@ config POWER_RESET_RESTART
 config POWER_RESET_VEXPRESS
 	bool "ARM Versatile Express power-off and reset driver"
 	depends on ARM || ARM64
-	depends on POWER_RESET
+	depends on POWER_RESET && VEXPRESS_CONFIG
 	help
 	  Power off and reset support for the ARM Ltd. Versatile
 	  Express boards.