char: nvram: disable on ARM

/dev/nvram was never meant to be used alongside the RTC CMOS driver from
drivers/rtc as it already expose the NVRAM through another interface..
Anyway, the last defconfig to enable it properly was removed in 2010 so
prevent ARM users from selecting it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>