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

(cherry picked from commit 774a4b6a51532b1a63530973d5bf8d798bc16669)