dev/hwpmc: Fix the dmc620 MD4 macro

Add braces to the dmc620 MD4 macro to fix the GCC build.

Reviewed by: brooks, imp, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45266

(cherry picked from commit 131c1718c6331e87f139b42b4ad0e57b6a71ea44)