keymap: Add extra Alt Gr mapping for Brazillian Portuguese ABNT2 keyboards

This adds missing Alt Gr mappings for the keys "q", "w", "e" an "c" to
conform with ABNT2 standard.

PR: 256416
Submitted by: Neebz <vpguyrhpjltta@logicstreak.com>
Reviewed by: emaste
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D33801