watchdog: rave-sp: add NVMEM dependency

We can build this driver with or without NVMEM, but not built-in
when NVMEM is a loadable module:

drivers/watchdog/rave-sp-wdt.o: In function `rave_sp_wdt_probe':
rave-sp-wdt.c:(.text+0x27c): undefined reference to `nvmem_cell_get'
rave-sp-wdt.c:(.text+0x290): undefined reference to `nvmem_cell_read'
rave-sp-wdt.c:(.text+0x2c4): undefined reference to `nvmem_cell_put'

This adds a Kconfig dependency to enforce that.

Fixes: c3bb3334 ("watchdog: Add RAVE SP watchdog driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>