imx_i2c: Make ext_resources non-optional

EXT_RESOURCES have been introduced in 12-CURRENT and all supported
releases have it enabled in their kernel config.

MFC after:      1 month
Differential Revision:	https://reviews.freebsd.org/D33830

sys/arm/freescale/imx/imx_i2c.c

@@ -73,7 +73,7 @@ __FBSDID("$FreeBSD$");
 #include <dev/fdt/fdt_pinctrl.h>
 #include <dev/gpio/gpiobusvar.h>
 
-#if defined(EXT_RESOURCES) && defined(__aarch64__)
+#if defined(__aarch64__)
 #define	IMX_ENABLE_CLOCKS
 #endif