imx_gpio: 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/D33831

(cherry picked from commit 90a5fc326e6484f02f9889ea7a356342f36731f4)