uart_dev_snps: 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/D33828

(cherry picked from commit 5c00765bf2378c8ed92abe78028f203e91ddbf91)