uart_dev_imx: 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/D33826

(cherry picked from commit 4f64df4e32a7c14c5862b5f866bae2b090d25edb)