power: reset: nvmem-reboot-mode: add CONFIG_OF dependency

Without CONFIG_OF, we get a build failure in the reboot-mode
implementation:

drivers/power/reset/reboot-mode.c: In function 'reboot_mode_register':
drivers/power/reset/reboot-mode.c:72:2: error: implicit declaration of function 'for_each_property_of_node'; did you mean 'for_each_child_of_node'? [-Werror=implicit-function-declaration]
  for_each_property_of_node(np, prop) {

Add a Kconfig dependency like we have for the other users of
CONFIG_REBOOT_MODE.

Fixes: 7a78a7f769 ("power: reset: nvmem-reboot-mode: use NVMEM as reboot mode write interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/reset/Kconfig

@@ -247,6 +247,7 @@ config POWER_RESET_SC27XX
 
 config NVMEM_REBOOT_MODE
 	tristate "Generic NVMEM reboot mode driver"
+	depends on OF
 	select REBOOT_MODE
 	help
 	  Say y here will enable reboot mode driver. This will