ARM: imx: reset_controller may be disabled

The new reset controller API is optional, so if that is disabled,
we must not call it from platform code.

arch/arm/mach-imx/built-in.o: In function
 `imx_src_init': /git/arm-soc/arch/arm/mach-imx/src.c:144:
	undefined reference to `reset_controller_register'

Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>