armv7: allwinner: Make ext_resources non-optional

The only valid condition for if_awg and aw_syscon is syscon.
EXT_RESOURCES is enabled in GENERIC since 12-CURRENT.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33833

(cherry picked from commit ee2e56c6d7f604e7aab8a8414753951a5809a985)